Let group inherit from Identity

* group.author is now group.admin
* group.groupname is now group.username
Add members attribute to groups
Add config LOG_SQL_STATEMENTS
Sync groups

* Persona.export includes groups
* Persona.(create|update)_from_changeset auto-requests new groups
* Fix Persona contact synchronisation
* Group exports/imports members
* Removed group_created signal
* Send local_model_changed signal for admin after group creation