Use image sources from the fileRepo API

This commit switches from using hard-coded media source urls to utilizing
MediaWiki's API call for meta=filerepoinfo. The API call is done only if
needed when the media insertion dialog is loading, and is done only once
per session.


Bug: 50673
Change-Id: Ia5ad9a8c00cca6cbbbc890359dc529e29e1a6be7
Merge "Use image sources from the fileRepo API"