admin: Inform about hidden items in details

Display the following stats at the start of each association:

* Number of hidden deleted entries.
  Note: if the administrator has specifically asked to show deleted
  entries, then this number will be zero, even if deleted entries are
  present
* Number of entries shown vs. number of total entries.
  The number of shown entries is always smaller or equal than the limit
  in the settings.

The underlying code for details has been largely revamped to allow
showing the above.
Merge branch 'feature-admin-details' into feature-admin