Configurable soft limit of 7,500, and above that, configurable
ratio of 1.1 * followers, controlled by:

- MAX_FOLLOWS_THRESHOLD
- MAX_FOLLOWS_RATIO

Fix #2311

* Add Follow#revoke_request!

* Implement Undo { Accept { Follow } } (fixes #8234)

Introduce recent to Follow, as Account and other models have.
This change also adds specs for the scope and the dependents.

StreamEntry model. Simply render Salmon slaps as they are needed

and atom feed generation to some extent, as well as the way mentions are
stored