Bug 50523: Hard-code Wikipedia rather than MW defaults

Our main user is Wikipedia, which uses 220px thumbnails by default rather than
MediaWiki's 200px. A proper fix will involve exposing this information
through the MediaWiki API, as discussed in bug 50523.

Change-Id: I02b9fb55324001f4001cd34bf31e0504d7661e25
Merge "Bug 50523: Hard-code Wikipedia rather than MW defaults"