Notification follow button style updates

 * Added follow button style helper to WPStyleGuide to fix some DRY.
 * Removed now unused FollowButton class.
 * Improved layout of follow NotificationsFollowTableViewCell to better match with other TableViewCells in the app (font, font color, white background).
Deleting old FollowButton class as well as pile of old images from the iOS 6 version.
Removing now-unused blogTitle String.
Merge branch 'develop' into issue/1052-consistent-follow-button

Conflicts:
	WordPress/Classes/FollowButton.h
	WordPress/Classes/FollowButton.m
Merge pull request #1535 from wordpress-mobile/issue/1052-consistent-follow-button

Notification follow button style updates