Add core event to functions_posting to modify notifications

Allows modification of notification data before it is processed into the database.  Useful for if you want to change any variables in the notification message, such as usernames.  Includes some other residual data to allow this event to be versatile.

I have an extension where I need to change peoples' usernames if they want to remain anonymous when posting.  Notifications are tricky, as of now I can update the "Quoted by ....." messages, but without access to the actual variables.  I opted to instead update the post_username of the notification in the notifications table, and this worked just as well.  These results are reflected in the manage notifications page in the ucp, too.