Comment moderation improvements

Improves comment moderation in both `NotificationsCommentDetailView` and `CommentDetailView`:

 * Adds text labels beneath the moderation buttons.
 * Replaces old, confusing icons with easier to understand versions (thumbs up/down for approve/unapprove and stop sign for spam). This matches up with WPAndroid.
 * Moves comment timestamp into the header area instead of next to the moderation icons.
 * An activity indicator is now displayed within the tapped moderation button, and all other moderation icons are disabled until the operation completes.
 * An alert view is shown when a moderation operation fails.
 * Tapping on trash now shows a confirmation action sheet in notifications (which matches up with comments).

Video: https://cloudup.com/chskLpNLCrQ

Fixes #79 
Same comment as https://github.com/wordpress-mobile/WordPress-iOS/pull/2134/files#discussion_r15712357