Support composing messages to recipients which include user-groups

[neiljp]

In the web app there are various places, including composing messages, where a user-group can be added (or auto-completed/selected).

For composition in the web-app, this results in the user group name expanding into all the members, which is likely the approach we'd want to consider first - though possibly considering cases such as a high number of group members given potential limited screen space.