This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
when configured by the admin a group can be found through the @mentions
feature in both the compose/reply and the private message user-selectors
and once selected the mention will be replaced by the list of users in
the group