[!!!] Add field to fe_groups to show relation between fe_users & fe_groups

* Needs DB Update in Extension manager
* Needs the following manual SQL update:

UPDATE fe_groups, fe_users
SET fe_groups.tx_filelinks_feuser = fe_users.uid
WHERE fe_users.usergroup = fe_groups.uid AND fe_users.deleted=0;

Closes #8 (basecamp)