Provide Atom feed for notifications.

[RokeJulianLockhart]

I can't feasibly keep re-enabling web notifications every time I reinstall my browser(/OS) or switch to a new device, so in lieu of the work necessary to setup and maintain e-mail notifications, could you instead provide an Atom feed? Atom is the standardized successor to RSS (it's basically identical except that it works - see bookwyrm-social/bookwyrm#3221 (comment) for a comprehensive explanation).