Comments/Notifications activities upgraded to use 'undo bar'.

 * CommentsActivity and NotificationsActivity now handle the moderation/reply actions instead of `CommentDetailFragment`.
 * Moderating a comment will dismiss `CommentDetailFragment` and show the undo bar for destructive actions.
 * Undo basically hides the comment that's being moderated until after the undo bar disappears, then the request is made. Tapping undo 'unhides' the comment/notification.
 * `NotificationsActivity` and `CommentsActivity` have both had the tablet landscape dual-pane mode removed.