Completely remove comment action types.

Users aren't really able to decide whether they just want to post a
comment or report a problem.  Practice showed that the "problem" action
was used more or less randomly.  So getting rid of it completely.

Removed the whole action-combo and related HTML.  Incorporated some of
the help text into the general commenting help section.

To clean up "action" field from database, run:

    db.comments.update({}, {$unset: {action: 1}}, false, true)