#490650 by litwol, JirkaRybka, nbz, Berdir, Liam McDermott: Message to oneself AND aditional recipients remains as unread forever. This patch fixes this behavior moving forward as well as providing update hook to update old messages that were affected by this issue.

.module: Remove subscription (and mail_edit) code which has been moved to pm_subscriptions.
.info: Remove dependency on subscriptions and mail_edit modules.
.install: Remove mail_edit queries. Roll back user table alteration. Roll back triggers update.

#182369: Add variables column in _install; Add update.

As this is a development branch, no rollback update path will be provided for testers. See issues in question for more information.

*DB upgrade*