Add a way to specifically allow-list remote domains/users for federation

[mayel]

Useful for both allow-list and consent-based federation, related to #1037

This could be done with dedicated circles (one for the instance + one for each user) and extra option in relevant dropdowns to add to them. These would probably only be used for allow-listed-only or consent-based federation.

But as a starting point, since we already store a list of known instances and a list of known actors, we can re-use those, which needs simply:

• [x] page to browse known instance in settings
• [x] ways to block actors/instances already on the list (related to #195 and #199)
• [x] option to delete a known actor
• [ ] a new federation mode setting for allow-list federation (related to #1039)
• [ ] checking that setting and allow lists before accepting activities from an actor/instance, or before fetching manually
• [x] a way for mods to lookup and add new actors/instances to the list
• [ ] option to delete a known instance (and all its users) (related to #1011)
• [ ] view lists of remote actors/instances filtered by blocked or not