Allow Meta Boxes to be added/removed per-Post Type

[d4mation]

This would tie in nicely with #177 if this were to become a separate extension/part of Pro

The idea would be to allow things like the Category meta box or perhaps ones added by a Plugin/Theme to be hidden per-Post Type and per-Role.

The Customizer can technically view different pages within the Admin, so the Preview could still be utilized in this way as well.

The main things we'd need to look out for are:

1. Ensuring the Meta Box is actually removed, rather than hidden
2. Ensuring that removing the Meta Box doesn't cause Categories or other things to be removed from the Post on Save
   • This can unfortunately be Plugin/Theme specific, as poor programming could have it assume the value should be deleted if it isn't passed via $_POST instead of just bailing instead as it really should. We will account for built-in Core stuff rather than catering per-Plugin/Theme.
3. Ensuring that this works in both the Classic Editor and Gutenberg

https://secure.helpscout.net/conversation/947032373/2404/?folderId=955389