Parse @-topic mentions

[gnprice]

When we get a message with an @-topic mention, we should parse the mention and show it in the message list. Currently it appears as "unimplemented".

That's this Zulip feature: https://zulip.com/help/mention-a-user-or-group#mention-all-topic-participants

The appearance should be the same as other wildcard mentions (@-channel, @-stream, @-all, @-everyone).

Umbrella issue:

• #817

Out of scope

• Separately we'll offer these in autocomplete, as part of (or a follow-up to): #234
• Separately we'll recognize the corresponding per-user message flag topic_wildcard_mentioned.
• Later we'll display these differently from mentions of an individual user: #646
• Later we'll highlight the mention, and the message as a whole, if it refers to the self-user: https://github.com/zulip/zulip-flutter/issues/647