Media query (grid and responsive utilities) overhaul

Grid classes have been changed to account for a fourth set of classes,
meaning we now have XS, S, M, and L options. Specifically, we made the
following changes:

* Renames `.col-` to `.col-xs-`
* Adds `.col-md-` for devices 992px wide and up
* Remaps `.col-lg-` for devices 1200px wide and up

Alongside that, we've updated our media queries to better handle
advanced grid layouts:

* All column classes now kick in when they hit a `min-width`, undoing
the #9128 fix in
db45a60
c690f3a65
* All column offsets, pushes, and pulls are scoped to an appropriate
range, meaning a `min-width` and `max-width` (thanks for the idea,
@eratzlaff)

We've also modified the widths of our grid containers for the small
device grid (now a max 720px instead of 728px) and large device grid
(now a max 1140px instead of 1170px) to avoid horizontal scrollbar
problems in Firefox (likely due to box-sizing, which is why we didn't
see it in 2.x).

Similarly, we've updated the responsive ultility classes to match the
new four media query approach of the grid system. That means we've:

* Added new `.visible-xs` and `.hidden-xs` classes
* Reassigns visible and hidden classes for small, medium, and large to
matching grid system media queries
* Updates docs examples to match grid system and account for fourth
utility range

-----

Fixes the following:

* #9135: fourth grid tier
* #9302: undoes previous range change for small grid system, so no need
to update any docs here
* #8755: consistent grid and responsive utilities schemas)
* #9195, #9216, and #9227: no more horizontal scrollbars

Author: mdo

_layouts/default.html

@@ -25,7 +25,7 @@ <h1>{{ page.title }}</h1>
 
     <div class="container bs-docs-container">
       <div class="row">
-        <div class="col-lg-3">
+        <div class="col-md-3">
           <aside class="bs-sidebar" role="complementary">
             <ul class="nav bs-sidenav">
               {% if page.slug == "getting-started" %}
@@ -42,7 +42,7 @@ <h1>{{ page.title }}</h1>
             </ul>
           </aside>
         </div>
-        <main class="col-lg-9" role="main">
+        <main class="col-md-9" role="main">
           {{ content }}
         </main>
       </div>

assets/css/docs.css

@@ -637,17 +637,9 @@ body {
 ------------------------- */
 .responsive-utilities-test {
   margin-top: 5px;
-  padding-left: 0;
-  list-style: none;
-  overflow: hidden; /* clear floats */
-}
-.responsive-utilities-test li {
-  position: relative;
-  float: left;
-  width: 25%;
 }
-.responsive-utilities-test li + li {
-  margin-left: 10px;
+.responsive-utilities-test .col-xs-6 {
+  margin-bottom: 10px;
 }
 .responsive-utilities-test span {
   padding: 15px 10px;