Automatically notify beatmap owners of new posts in their difficulty

[syzyxy]

This is so they can easily apply mods, no longer needing to DM them on discord or whatnot. Also useful for collabs so they can check right away if a mod is from their part or not. Using the 'watch' feature isn't really ideal since it notifies every post in the map discussion.

This change would save so much time for both map hosts and GDers. This is essentially receiving notifications, similar to how the current system notifies the map host about new posts, so hopefully it should be quite easy to implement.

[peppy]

Seems like something which should/could happen, for sure.