FEATURE: Actually show more notifications

The "Show more notifications..." link in the notifications dropdown now
links to /my/notifications, which is a historical view of all
notifications you have recieved.

Notification history is loaded in blocks of 60 at a time.

Admins can see others' notification history. (This was requested for
'debugging purposes', though that's what impersonation is for, IMO.)
Final dirty diff cleanup
Merge pull request #2743 from riking/notification-history

FEATURE: Actually show more notifications