Improve logic for marking at-mention notifications as read

In previous iterations, at-mention notifications were marked as read in the
following cases:

- when a user visited his own activity page
- when a user visited his own mention page
- when a user visited a single-activity permalink page

When the new notifications component was introduced in BP 1.9, the old
notification functions were swapped out one-for-one with the new correlates.
However, this introduced a bug where a logged-out user visiting one of the
pages listed above would mark at-mention items as read for *all* users (due to
the user_id field being null, and the user_id WHERE clause being excluded from
the UPDATE query).

We take this bug as an opportunity to narrow the circumstances in which
at-mention notifications are marked as read. The new system:

- When a user visits his own mention page, mark all his at-mention notifications
  as read
- When a user visits the permalink page of a single activity item in which he
  is mentioned, mark that specific at-mention notification as read

Fixes #5384

Props SlothLoveChunk for an initial patch

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7831 cdf35c40-ae34-48e0-9cc9-0c9da1808c22