Setup/restore #mediawiki-feed

Relay all MediaWiki-related changes also to this (abandoned) channel.
Exclude repos not depending from MediaWiki development which used to
have their own repos and commits/bugs lists: pywikibot and operations.

I5737320a5510b7d647f8ff79a6f2ba045ef34774 proves this should work.

Bug: 52529
Change-Id: I89de0279ab52c3fa5367c0bb3e60853c3281b791