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:

• page to browse known instance in settings
• ways to block actors/instances already on the list (related to Silence an entire remote instance (as a user)  #195 and Silence a local user (instance-wide by admin)  #199)
• option to delete a known actor
• a new federation mode setting for allow-list federation (related to Add federation modes: temporarily paused, allow-listed-only & consent-based #1039)
• checking that setting and allow lists before accepting activities from an actor/instance, or before fetching manually
• 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 'delete contents and block' feature for mods #1011)
• view lists of remote actors/instances filtered by blocked or not