Changes/improvements regarding Sign-in IP recording

[WilliamWDTK]

Consider:

• Not making it immediately visible on user pages (see Update privacy page regarding retention of and access to IP addresses for sign-ins whatdotheyknow-theme#1203)
• Allowing sorting of the overview list by number of other users on the IP (ascending/descending)
• Perhaps making some privacy changes might be necessary to Alaveteli itself, not just site-specific privacy pages (although I assume it is disabled by default…)
• Whether it is necessary to show multiple sessions with the same user—IP combination
• Adding a message where no sessions are registered explaining that the sessions might be older than x days, rather than just that there are none:
  
  Could even mention the retention period even if sessions are recorded…

These aren't necessarily firm suggestions, just a few thoughts that came to me, including by reading recent WDTK internal correspondence on the subject, as well as the linked ticket.

See #6822

[WilliamWDTK]

Perhaps also for the main overview page, hiding records where no other users are signed-in on the same IP address by default.

[garethrees]

Not making it immediately visible on user pages

Done in #7592.