UI for adding and removing change tags on revisions and log entries

UI for adding and removing change tags on revisions and log entries

There is a new special page, Special:EditTags, which is very similar to
Special:RevisionDelete in a lot of ways. In fact, the SpecialEditTags class
started off as a copy-paste of SpecialRevisiondelete.

You invoke this special page by going to an article history page, checking
some revisions, and clicking "Edit tags of selected revisions". Then you
pick the modifications you want to make and click "Apply". Very much like
the revision deletion workflow.

I had to restructure some of the Action routing code, which was only
designed to handle revision deletion. Also removing some code from
SpecialRevisiondelete which didn't work as advertised in the first place,
and definitely doesn't work now.

Change-Id: I7d3ef927b5686f6211bc5817776286ead19d916b