autocomplete: Support wildcard @-mentions

[chrisbobbe]

With #207 being merged, we now have an @-mention autocomplete UI for user mentions! 🎉

We should extend it to support autocompleting wildcard mentions, like @**all** and @**channel**.

The exact set of wildcard-mention options will depend on the server version:

• "channel" being an option for new servers, and "stream" for old servers — due to Rename "stream" to "channel" zulip#28468. See ui: Use "channel" or "stream" based on server feature level zulip-mobile#5830 for how we handle that in zulip-mobile.
• "topic" being an option for servers 8+ but not older servers; see compose: Enable @-topic for servers that support it zulip-mobile#5792