Add the ability to save/subscribe to beatmap searches

[smileyhead]

A suggestion. It would basically be the osu! version of the YouTube notification bell. This has to do with osu!lazer as well, but I thought it would be more appropriate to post here.

• Following users: there could be a Follow button on a user's profile, pressing it will place the user on your list.
• Following searches: when you perform a search, there could be a Follow button on the results page that places that set of keywords on your list. I'm not sure if these are a thing already, but if not, you could introduce some search tags too, like 'artist:' and 'source:', so people can follow specific artists and sources.

On startup, osu!lazer could check if there are any new mapsets by the creators or with the terms you follow, and push a notification if so. Maybe there could be a new screen listing all the new mapsets, so you don't get bombarded with a separate notification for every mapset. Either that, or have said new screen accessible from somewhere in the title screen that lists all the mapsets by your followed creators/keywords in chronological order and marks the new ones.

[Joehuu]

Notifying new per-user beatmaps added in https://github.com/ppy/osu-web/pull/6909. Would like the second half implemented too.