feature #280502 followup by greggles: let the admin choose which modes are enabled in case, for example, they don't use threaded comments

feature #280502 by greggles: optionally only send notifications for replies to the specific comment.

bug #317638 by Junyor: Thinko in the Pgsql case - should be creating table comment_notify, not re-creating the comments table round 2

bug #317638 by Junyor: Thinko in the Pgsql case - should be creating table comment_notify, not re-creating the comments table

bug #305259 by greggles: if a user can't subscribe to comments don't add form elements to their user edit screen

feature #296549 by greggles idea by bchoc: if anonymous can't leave an email, hide the checkbox / provide help to admins to get the settings right

feature #171034 by greggles: if emails are not required by drupal, validate the comment submit form to require emails if they try to get comment notification

feature #229582 by greggles: Don't notify registered users by default - let them opt-in and provide help to change the default.

feature #232564 by greggles: refactor data structure so core tables are not altered and move towards PostgreSQL compliance