[ticket/15883] Use the new language object
Using the new ->lang() format. Made the username
comparison case insensitive. Also fixed a bug where the return to
previous page link in the UCP was going back to the list of
groups rather than the specific manage group page when an error
occurred.
[ticket/15883] Add error for invalid usernames being added to a group
Update the ACP and the UCP so that when bulk adding users to a group,
if invalid usernames are submitted alongside valid usernames then a
message will be displayed to inform the user what the invalid usernames
are.
