Report reasons: fade out on message report

This is a tag onto reddit/reddit@85a9223 and reddit/reddit@76858eb99.  Report
reasons weren't originally supported for PMs, so we returned early and didn't
call `form.fadeOut()`.  Now that we use the report reasons box everywhere, we
need to handle the form for everything, too.

[Reported][0] by /u/psdtwk.

[0]: https://www.reddit.com/r/bugs/comments/2p6m56/

r2/r2/controllers/api.py

@@ -1557,7 +1557,7 @@ class ApiController(RedditController):
             Report.new(c.user, thing, reason)
             admintools.report(thing)
 
-        if isinstance(thing, Link):
+        if isinstance(thing, (Link, Message)):
             button = jquery(".id-%s .report-button" % thing._fullname)
         elif isinstance(thing, Comment):
             button = jquery(".id-%s .entry:first .report-button" % thing._fullname)