[PHPBB-14638] New event hooks for the UCP subscriptions page

[rfdy]

Forgive me if this isn't big enough for an RFC, I figured it'd be better to err on the side of caution. This ticket is to add six new hooks to the ucp_main module that allow us to manipulate the SQL queries to grab lists of subscribed forums and subscribed/bookmarked topics, as well as the template variables phpBB generates from the resulting data.

The reason we want to add these hooks specifically is to support an extension we're building to support per-topic and per-forum subscription notification settings. The extension would allow users to, for example, only get email notifications for certain threads but not board notifications, or vice-versa. That said, the hooks have a general enough purpose that I could see them being used for other things as well (ex. storing additional information with subscriptions and displaying them to the user, or additional filtering on subscribed topics/forums).

I've posted a pull request with the new hooks. If there's anything else I should do to facilitate discussion, please let me know. Thanks.