Rewrite notifications.
Remove IndirectReply, merge Mention into Notification.NewPost.
Rename file, move class.
Fix test suite compilation errors.
Add notification generator tests, fix issues

This requires making some methods non-final so they can be mocked.
Make NotificationGenerator generate mentions.
Delete notifications, if post deleted, or edited and mentions removed.
Fix notification HTML rendering so emails work again.
Change emailCreatedAt to emailStatus.
Generate notifications to people watching a page.
Merge branch 'topic-good-notifications'.