Add a "mention" notification

Any user whose user page is linked in a comment on a talk page will get a notification of that.

Weaknesses: Currently this mention notification is additive.
We may want to restrict it to only cases where the user would not
otherwise be notified of the comment

patch set 3:
	* user + instead of array_merge for merging subscription users
	* rename $user to $agent to avoid name confilict in generateMentionEvents()
	* add check for possible null object
	* users should not receive 'mention' notification on their own talk pages

patch set 4:
	* add more descriptive comment
	* check for empty notification list before creating mention event

patch set 5:
	* Fix a parse error, change [ to {

patch set 10:
	* rebase

patch set 11:
	* adding flyout messages, updating params for other messages

Change-Id: I76b80db1f325d9569f36c506d14c8c875bba4a34
Merge "Add a "mention" notification"