Handle moderation privileges - 2nd go

Supersedes !209 (closed)
For BE MR pleroma!3676 (closed)

The idea in !209 (closed) was to have fine grained control of what we show in admin-fe. With a minimum of privileges, you'd be able to already do things. Things you're unable to do would be hidden in the pages. The problem is that this turned out to be much more work than anticipated, partly because admin-fe also does several calls which are unexpected and fail if you don't have the right privileges.

Admin-fe doesn't seem like it was ever designed to handle this. I think it needs more redesign work than what I'm currently capable of doing. This together with talks of moving away from admin-fe, makes it not seem worth the effort.

In the end I decided to make this MR. Instead of going super fine-grained on the admin-fe side, I only show menus on the right when you're privileged for everything the menu allows.