Make it possible to add a button when creating pdf links #172
Description
When creating or editing a link, it should be possible to add a button.
In the left bar, add a "Button" input, listing all resources available for the publication. If the user selects one of them, add the argument wabutton=%20[Publication Folder]%20[Name of file]. Also, make it possible to upload a new image, if the user has not done so before.
When the user has selected a button, the corresponding image should be displayed in the preview, and in the reader.