Feature: user-level whitelist
Adds a user->user "whitelist" relationship that overrides domain blocks for single users
use case is as follows:
- user massively dislikes an instance
- but there are one or two users on that instance that the user wants to speak to
- user cannot domain block without losing access to those few people they want to talk to
- hence there needs to be a way to create exceptions to the blanket rule
adds the following endpoints
POST /api/v1/pleroma/accounts/:id/whitelist
POST /api/v1/pleroma/accounts/:id/unwhitelist
proposed order of precedence: block > whitelist > domain block