Performance improvements and fixing the groups code

References #162 and #163 

This one is working for me in my staging environment now.

Master branch of Quickmail had an execution time of around 13 seconds. With the first round of fixes for the groups now being in SQL, and having fullname overridden, it came down to around 6 seconds. Enabling the studentsonly feature brought it down to 2 seconds.

I think this one now should work better among different DB engines as group_concat is now removed.

The experimental settings are now also in a collapsed fieldset on the config page so they are not readily visible.

Let me know your thoughts.