Fix group permission fixture
Allow general manager to access the gym member list

This is restricted to admin users
Use permission groups for gym administrators

Using permission groups 'gym_trainer', 'gym_manager' and 'general_gym_manager'
instead of assigning permissions directly to gym administrators
Show last activity on gym member list

The last activity is the most recent of either workout log or workout sessions.
Additionally, the table is made sortable using the jQuery plugin dataTables.