Attempt to count actual watchers in the info action

Attempt to count actual watchers in the info action

Proposed threshold to be considered an "active" watcher:
two times $wgRCMaxAge, configurable with the new
configuration setting $wgWatchersMaxAge.

The information is not displayed when the number given would
be 1 or 0, so that the number (or absence thereof) doesn't
"disclose" that the page is (potentially) unpatrolled or
completely unwatched and hence easier to vandalise.
Configurable with $wgUnwatchedPageSecret too.
Also, we don't display this row at all when the user doesn't
have the right to see the count of total watchers.

Bug: T51506
Change-Id: I10d294a339b131eee94839ed7088ab20d746d881