ChangeTags: Hide tags whose description messages are disabled

If an interface message describing the given tag exists, check if it has been
disabled. If so, take that as an indication that the tag should not be
displayed.

This amounts to an adequate (if somewhat low-level) interface for managing
which edit tags are shown in change list views like Special:RecentChanges and
Special:Contributions.

This patch is based on a suggestion made by jackmcbarn in a review comment on
commit 494989772cf. I did it in a separate change because I'd be interested in
hiding the HHVM tag even if we end up deciding against a generic mechanism for
hiding tags.

Change-Id: I515cf2118ab929da985d6b40481325640e800dcd