msglist: Show recipient headers on all messages, in Mentions / Starred

[chrisbobbe]

#mobile > search @ 💬

Every message [in search] should show a recipient header, even if it's to the same conversation as the one before it (because there might be other non-matching messages in between those two, so we shouldn't give the impression that one was immediately after the other).
This just occurred to me in thinking about search, but I think it should also apply to the starred-messages and mentions views, and is a bug we currently have in those. Not hard to fix.

Related: #252