Enable users to watch category membership changes #2

Enable users to watch category membership changes #2

This is part of a chain that reverts:
e412ff5ecc900991cce4f99b7a069f625a5694b3.

NOTE:
- The feature is disabled by default
- User settings default to hiding changes
- T109707 Touching a file on wikisource adds and
      removes it from a category... Even when page
      has no changes.... WTF? See linked issue,
      marked as stalled with a possible way forward
      for this patch.
      @see https://gerrit.wikimedia.org/r/#/c/235467/

Changes since version 1:
- T109604 - Page names in comment are no longer
      url encoded / have _'s
- T109638 & T110338 - Reserved username now used
      when we can't determine a username for the change
      (we could perhaps set the user and id to be blank
      in the RC table, but who knows what this might do)
- T109688 - History links are now disabled in RC....
      (could be fine for the introduction and worked
      on more in the future)
- Categorization changes are now always patrolled
- Touching on T109672 in this change emails will never
      be sent regarding categorization changes. (this
      can of course be changed in a followup)
- Added $wgRCWatchCategoryMembership defaulting to true
      for enabling / disabling the feature
- T109700 - for cases when no revision was retrieved
      for a category change set the bot flag to true.
      This means all changes caused by parser functions
      & Lua will be marked as bot, as will changes that
      cant find their revision due to slave lag..

Bug: T9148
Bug: T109604
Bug: T109638
Bug: T109688
Bug: T109700
Bug: T110338
Bug: T110340
Change-Id: I51c2c1254de862f24a26ef9dbbf027c6c83e9063