Add group-like actor for people to mention to get reviews from Mastodon onto BookWyrm instances

[Skivling]

Is your feature request related to a problem? Please describe.
Related to #3501, it is related to the problem that there are many book reviews happening on other fediverse platforms like Mastodon, but many will never be federated onto BookWyrm instances.

Describe the solution you'd like
Each instance would have an Actor named @[email protected] or something similar (of course different if that's already a username). These would function the same as fediverse groups do, boosting all posts sent to them and being followed to receive those posts. They also provide the ability for book review posts to become discovered by BookWyrm instances (and those which are federated to each other) so they can be integrated into BookWyrm as described in #3501.

Instances federate together in some way so that viewing the book reviews on https://ramblingreaders.org will show reviews which were sent to https://bookwyrm.social 's bookreviewers group. I don't know enough about this to know how it would work technically.

If this is implemented and publicised well, especially with the community at #Bookstodon and other book communities in the fediverse, it will become normal to add it to book review posts and everyone benefits.