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 
Can you change this (and other case in the PR) to only use dot notation rather than mixing both dot and square brackets together?  Dot notation is cleaner and easier to read, IMO.