admin: Update RBAC settings

The RBAC policy has now a default value that is stored in
admin_settings.py. The user however can use the admin conf file to
tailor to his/her needs.

The user can either change the default three groups (admin, helpdesk,
admin-readonly), or create a new ADMIN_RBAC.
admin: Add settings to hide deleted items

Add ADMIN_SHOW_DELETED_ASSOCIATED_ITEMS settings which defaults to
False.

With this option, the user can define whether to show deleted items on
the details page of another item. Note that the details page of the
deleted item will be shown properly.
admin: Add setting to limit associated items

Add ADMIN_LIMIT_ASSOCIATED_ITEMS_PER_CATEGORY setting which defaults to
50.

With this option, the user can define the number of associated items
that will be shown for each category, so as not to flood the page.
Merge branch 'feature-admin-settings' into feature-admin