Add "visibility" feature

[pfefferle]

Fix #923

This PR allows users to change the visibility of a post.

Options:

• Public -> default
• Quiet public -> This behaves exactly like public, except the post will not appear in live feeds or hashtags, explore, or Mastodon search, even if you are opted-in account-wide.
• Do not federate -> Do not federate this post.

Todo: Use constants

[mattwiebe]

Poked around a bit, had a few thoughts, but the React looks like a seasoned pro!

[pfefferle]

@mattwiebe

Poked around a bit, had a few thoughts, but the React looks like a seasoned pro!

😍

[mattwiebe]

A couple of questions:

1. Is this going to break Outbox pagination? Should we adjust the query there to exclude hidden posts?
2. Do we ever even want to save public? Should it be more of a default null case?

[pfefferle]

@mattwiebe

1. Mastodon does not really support the Outbox, so we could care about that later.
2. Good point. Will WordPress ignore the null value or will it persist the null?