Offer @-topic mentions in autocomplete

[gnprice]

This is a follow-up issue to:

• autocomplete: Support wildcard @-mentions #234

This issue has as a prerequisite:

• Parse @-topic mentions #892

Once we're parsing @-topic mentions when they appear in message content, we can start offering them in autocomplete in the compose box. (We don't want to do so before we're parsing them in content, because then the newly-sent message wouldn't render right, which isn't a good experience.)

Umbrella issue:

• Support @topic wildcard mentions #817