Improve admin (faster loading, restriction of foreign key & m2m choices)

• Remove inline editing on AKSlot admin page (Close #106 (closed)): This reduces the amount of queries from over 450 for a full list (100 entries) to about 50, resulting in a drop of execution time from about 20 seconds to less than one second
• Restrict choices (foreign keys & m2m) in admin forms to other objects of the event of the edited instance
• Improve how requirements are shown for AK admin (multiple checkboxes instead of multiselect widget)