Added support for purging backlinks in the wiki farm

* This adds a $wgGlobalUsageSharedRepoWiki and a $wgGlobalUsagePurgeBacklinks variable
* This works by pushing jobs on the repo wiki that push HTMLCacheUpdateJob jobs on all
  the client wikis for the imagelinks table

bug: 22390
Change-Id: Ib764e9774d208a8b20d4d67db229c24fd21734c8
Merge "Added support for purging backlinks in the wiki farm"