Add "Developers" link to footer of Wikimedia wikis

This change establishes two new messages:
* "MediaWiki:Wikimedia-developers"
* "MediaWiki:Wikimedia-developers-url"

The "-url" message can be an internal or external link. By default, it links
to <https://www.mediawiki.org/wiki/Special:MyLanguage/How_to_contribute>.

Bug: 33464
Change-Id: I7a35225bdb2ca78f292b94ae470ec8f85a998db7
Merge "Add "Developers" link to footer of Wikimedia wikis"