SimpleMRF: Let instances be silenced

This MR lets instances be "silenced", similar to Mastodon. Basically it only allows posts from that server to address users who follow the actor. Posts from these servers won't be shown in public timelines.

This is a good compromise when you don't want to reject messages from another server entirely, but you don't want users to see them by default.

EDIT: Rather than a "silence", the setting is now called [:mrf_simple, :followers_only]