[#397458] by dww: Massive edit of the code for consistent terminology.

- We use "notification" not "subscription" everywhere.
- The two new tables are now {project_issue_notification_global}
  and {project_issue_notification_project}. Both are per-user, the key
  difference is global settings vs. per-project settings.
- Renamed the constants for notification levels: we always want the
  constant to be "OWN", and just let the meaning of "OWN" change
  depending on whether or not flag.module is enabled and configured.
- Use "follow" terminology when appropriate for the flag.module case.
- Fixed PHPDoc to better conform to standards.
- Clean up the 'project_issue_follow_flag' setting on uninstall.
- Renamed includes/subscribe.inc to includes/notification.inc.