Add two more functions - group_param_{group,user}id, which replace a whole lot of duplicated validation in lib/group.php.

Previx get_grouptype_options function with group_. Added some miscellaneous comments to describe how lib/group.php is ordered.

Perform validation on the parameters while we're in the area.

May as well have one, given we have group_delete.

Takes a data structure that describes the new group, and checks that the new group will be created in a sane way.

Replace delete_group with a new function, group_delete. Same implementation, but has a doc comment and parameter checking.

Add doc comment to group_user_can_edit_views. Correct error messages for the InvalidArgumentExceptions.

Now uses $userid/$groupid, they're checked for validity, and userid is optional.

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.

Unconditionally redirect after submitting the changerole form, which prevents crashing if someone attempts to submit the form without changing the role.

This is done using a form, although it's styled like all the other delete links we have around the place.

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.

Add an empty TD into the members table if there's an odd number of results, which fills in the table background and makes it valid HTML.

Move the 'site closed' message to being at the top of the page, where it doesn't interfere with the menus in the admin section.

Reuse the same styling that the 'masquerade' stuff has.