Make admin simply a boolean instead of group

This simplifies a lot of logic. Keeping an admin group has no benefit

src/groupdb.js

@@ -152,7 +152,7 @@ function clear(callback) {
     database.query('DELETE FROM groupMembers', function (error) {
         if (error) return callback(new DatabaseError(DatabaseError.INTERNAL_ERROR, error));
 
-        database.query('DELETE FROM groups WHERE id != ?', [ 'admin' ], function (error) {
+        database.query('DELETE FROM groups', function (error) {
             if (error) return callback(new DatabaseError(DatabaseError.INTERNAL_ERROR, error));
 
             callback(error);