This also adds a database column to store the setting.

Whether this setting is even settable is controled by the site setting added in the previous commit.

Changes to each group file to make them visible to the public are coming in the next commit.

message when inviting users to groups.

Prevent role changing in groups if the user is the only admin. Add some functions to abstract role changing.

This patch adds:

 * group_is_only_admin($group, $user)
 * group_can_change_role($group, $user, $newrole)
 * group_change_role($group, $user, $newrole)

These functions are used on changerole.php, and also on the member listing page to prevent showing the change role link if the user is the only admin in the group.

If the user has no roles that they can change to and changerole.php is visited, it will redirect back to the member listing with an informational message.

Prevent users from leaving a group (in group_remove_user) when they are the only admin in the group.

Add caching to group_user_can_leave.

Prevent a crash when a group has no forum posts in it. Fix up some language strings on the group info page too.

The name is largely irrelevant as it's only the creator of the file.

Ran the copyright notice corrector script so all Mahara related files are correctly licensed under the GPL3.