Renaming NotificationsBigBadgeViewController to be more consistent.

The other notification detail controllers included `Detail` in the
class name's(i.e. NotificationsFollowDetailViewController) whereas
NotificationsBigBadgeViewController did not. Renamed it to be more
consistent.
Merge pull request #2063 from wordpress-mobile/issue/naming-cleanup

Renaming NotificationsBigBadgeViewController to be more consistent.