Support @topic wildcard mentions

[PIG208]

This Zulip feature (https://zulip.com/help/mention-a-user-or-group#mention-all-topic-participants).
API docs: https://zulip.com/api/update-message-flags#available-flags
Relevant discussions: https://chat.zulip.org/#narrow/stream/378-api-design/topic/highlight.20topic-wildcard.20mention/near/1621613

[gnprice]

This has a couple of different parts, which we might split into several issues when we start working on it.

• Understanding the new pair of flags for stream/channel vs. wildcard mentions. → Handle flags for @-topic mentions #894
• Offering these in autocomplete, as part of autocomplete: Support wildcard @-mentions #234
  → Offer @-topic mentions in autocomplete #893
• Parsing these when they appear in message content. → Parse @-topic mentions #892
• Highlighting these in the message list: content: Distinguish direct and wildcard mentions #646, content: Highlight non-silent mentions of the self-user #647.

[gnprice]

OK, I've split this up into those more-specific issues, and edited the comment above to point to them. Closing this umbrella issue in favor of using those to track the work.