Add support for user-group mentions

[timabbott]

This is basically the JS side implementation of this feature for the webapp:

zulip/zulip#6660

We should try to quickly integrate the display part; typeahead is less important but probably also a good idea. I expect we'll have a UI for creating these in a few weeks.

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edit:

This is the feature now documented here: https://zulipchat.com/help/user-groups

[chrisbobbe]

@timabbott, this looks pretty old, and the mobile app already has some support for group PMs; do you know off-hand if there's still work to be done here? You mentioned typeahead; if that's still pending, it may come soon with the shared code issue zulip/zulip#13728 and its mobile counterpart for the people typeahead. (Actually, it looks like we haven't filed a mobile issue for the people typeahead; I just opened that as #3902.)

[timabbott]

This feature is for the mentionable "user groups" feature, not group PMs. See https://zulipchat.com/help/user-groups.

[chrisbobbe]

Thanks, Tim!