Adds follower feedback in comments/replies as part of #270

Indicates whether or not any users or user groups were selected to receive a notification of a new comment (the first part of #270). Feedback is given both in the comment/reply form and in the comment itself (on hover, like the reply link). The indicator in the comment persists, for transparency's sake, when/if Edit Flow Notifications are turned off.

I split #270 up into two pull requests - I believe them to be two separate features.