Removing Toast Message duplication as a result of 799240ab9f.

In 799240ab9f @beaucollins added code for a toast message related to Following and Unfollowing blogs. I noticed that in NotificationsFollowDetailViewController some of the [code](https://github.com/wordpress-mobile/WordPress-iOS/blob/notifications/Classes/NotificationsFollowDetailViewController.m#L228) could be removed and take advantage this commit. I modified some of the code he added to be a little more generic(moved the notification constants into Constants.h and renamed them).