Support autocomplete of wildcard mentions

Mentions in general are summarized here: https://docs.zulip.com/help/mention-a-user-or-group

Many of the options are autocompleted as documented in our FAQ entry.

Current outstanding elements are:

• @**topic** (only feature level 224, Zulip 8?)
• @**stream** / @**all** / @**everyone** (feature level 247 adds channel, will be Zulip 9)

For feature-level-dependence, we should skip autocompleting any that do not match the server version having a sufficiently large feature level. The web app currently only shows the two shortest (available) matching wildcard options - but best to confirm with the web app behavior when implementing.

Implementation of this may depend on elements of #1164, to allow any matching wildcard mentions to be made distinct from stream/group mentions, or other related improvements. In a similar way, we should examine the order of web app autocomplete for comparison.

A connected feature would be to extend notifications, which also relates to some of these elements and could be improved substantially in #666.

This may need some discussion first, but a sample with tests should be well-defined by the above.