Fix user group not showing in an overlay if "Username styling for a mentioned group" is disabled
Prefix all permissions with '[UMI]'
Nail down a few more "tag group" to "mention group" references
Add '[UMI] View mentionable user group members' permission
By default granted to registered and unregistered usergroups.
Controls if a user group member can see the member list for public user groups
Global option for how completely this is hidden, by default it returns no permission. But can completely hide the user group id (returning a 404), and not render the user group mention bbcode.