Converted html cache updates to use BacklinkJobUtils

* De-duplicate squid purges using page_touched
* Removed excess title validity check
* Also removed broken versioning in refreshLinks jobs
* Added TODO comment about $wgMaxBacklinksInvalidate
* Other documentation tweaks

Change-Id: If91e06549e5afa2d4e27595d0e131b4d78412ad2
Merge "Converted html cache updates to use BacklinkJobUtils"