autocomplete: Support user-group @-mentions

[chrisbobbe]

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

We should extend it to support autocompleting user-group mentions, like @*mobile*.

[gnprice]

When implementing this, we should be sure to exclude deactivated groups from the results.

Until recently, such groups weren't included in the data the server gave us in the first place. Now they are:

• https://chat.zulip.org/#narrow/stream/378-api-design/topic/deactivated.20groups.20in.20.60GET.60.20endpoints/near/1940339
• Store user groups #662 (comment)
• user_groups: Support deactivating user groups. zulip#30504