Update class.discussionmodel.php

Check for permissions when notifying of new discussions
Partly fixing https://github.com/vanillaforums/Garden/issues/1518
Update class.commentmodel.php

Check permissions when using advanced notifications
Partly fixing https://github.com/vanillaforums/Garden/pull/1740
Merge pull request #2113 from vanilla/hotfix/notify-permission

Check permissions before new discussion notifications