This way if you refer to "trac #253" in the subject, it's super
convenient to get from your recipient bar to the ticket.
A note on performance: this part of rendering for 1000 messages takes
about 3.5ms for messages with 1 match; this is small compared to the
overall time for to_dict_uncached for that many message objects, so I
think this is OK for now.
(imported from commit 5bdc2b8415d7599d59eb554739f545c485b78d5a)