MRF TagPolicy engine

This MR contains an MRF policy module that selects behavior based on configured user tags.

These are:

• mrf_tag:media-force-nsfw: force attachments to be NSFW
• mrf_tag:media-strip: strips media uploads from posts
• mrf_tag:force-unlisted: public posts become unlisted
• mrf_tag:disable-remote-subscription: disable following the remote user
• mrf_tag:disable-any-subscription: disable following the user at all
• mrf_tag:sandbox: forces posts to be followers-only

Coming once federation policy hooks are decided, maybe this MR or not:

• mrf_tag:quarantine: disable outgoing federation for the user