Switch to https for all resources and external links

This avoid mixed content warnings in browsers.
Fixes spring-projects#7134

Author: bclozel

_config.yml

@@ -26,7 +26,7 @@ name: Spring Boot
 project: spring-boot
 
 # Project github URL
-github_repo_url: http://github.com/spring-projects/spring-boot
+github_repo_url: https://github.com/spring-projects/spring-boot
 
 # If you want to include a custom pom.xml or gradle template set these value to true and add _include files
 custom_pom_template: true
@@ -35,9 +35,9 @@ custom_gradle_template: true
 
 ### The following properties are constant for most projects
 
-main_site_url: http://spring.io
-projects_site_url: http://projects.spring.io
-questions_url: http://spring.io/questions
+main_site_url: https://spring.io
+projects_site_url: https://projects.spring.io
+questions_url: https://spring.io/questions
 
 version: 1.0.0.BUILD-SNAPSHOT
 exclude: [code]

index.html

@@ -22,7 +22,7 @@
       icon: ci
 
     - name: StackOverflow
-      url:  http://stackoverflow.com/questions/tagged/spring-boot
+      url:  https://stackoverflow.com/questions/tagged/spring-boot
       icon: stackoverflow
 
 #    - name: Metrics (SonarQube)
@@ -49,13 +49,13 @@
 * Provide production-ready features such as metrics, health checks and externalized configuration
 * Absolutely <strong>no code generation</strong> and <strong>no requirement for XML</strong> configuration
 
-The [reference guide](http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle) includes detailed descriptions of all the features, plus an extensive [howto](http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#howto) for common use cases.
+The [reference guide](https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle) includes detailed descriptions of all the features, plus an extensive [howto](https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#howto) for common use cases.
 
 <span id="quick-start"></span>
 
 ## Quick Start
 
-If you are Java developer you can use [start.spring.io](http://start.spring.io/) to generate a basic project, follow the "Quick Start" example below, or read the reference documentation [getting started](http://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#getting-started) guide.
+If you are Java developer you can use [start.spring.io](https://start.spring.io/) to generate a basic project, follow the "Quick Start" example below, or read the reference documentation [getting started](https://docs.spring.io/spring-boot/docs/current-SNAPSHOT/reference/htmlsingle/#getting-started) guide.
 
 {% include download_widget.md %}
 
@@ -88,20 +88,20 @@
 
 ## Spring Boot CLI
 
-Spring Boot ships with a command line tool that can be used if you want to quickly prototype with Spring. It allows you to run Groovy scripts, which means that you have a familiar Java-like syntax, without so much boilerplate code. Follow the instructions in our main documentation if you want to [install the Spring Boot CLI](http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#getting-started-installing-the-cli).
+Spring Boot ships with a command line tool that can be used if you want to quickly prototype with Spring. It allows you to run Groovy scripts, which means that you have a familiar Java-like syntax, without so much boilerplate code. Follow the instructions in our main documentation if you want to [install the Spring Boot CLI](https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#getting-started-installing-the-cli).
 
 {% endcapture %}
 
 {% capture related_resources %}
 
 ### Sample Projects
 
-* [Simple](http://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-simple)
-* [Embedded Tomcat](http://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-tomcat)
-* [Batch](http://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-batch)
-* [Data JPA](http://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-data-jpa)
-* [Integration](http://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-integration)
-* [more...](http://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples)
+* [Simple](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-simple)
+* [Embedded Tomcat](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-tomcat)
+* [Batch](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-batch)
+* [Data JPA](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-data-jpa)
+* [Integration](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples/spring-boot-sample-integration)
+* [more...](https://github.com/spring-projects/spring-boot/tree/master/spring-boot-samples)
 
 ### Getting Started Guides
 
@@ -113,9 +113,9 @@
 ### External Links
 
 * Presentation from [Spring One 2GX 2013](https://speakerdeck.com/dsyer/spring-boot-introduction)
-* InfoQ article on [Micro Frameworks](http://www.infoq.com/articles/microframeworks1-spring-boot)
-* InfoQ news on [release of 1.0.0](http://www.infoq.com/news/2014/04/spring-boot-goes-ga)
-* Josh Long's vJUG [session on YouTube](http://www.youtube.com/watch?v=eCos5VTtZoI) from April 2014
+* InfoQ article on [Micro Frameworks](https://www.infoq.com/articles/microframeworks1-spring-boot)
+* InfoQ news on [release of 1.0.0](https://www.infoq.com/news/2014/04/spring-boot-goes-ga)
+* Josh Long's vJUG [session on YouTube](https://www.youtube.com/watch?v=eCos5VTtZoI) from April 2014
 {% endcapture %}