Only allow h3-h6 when saving posts. Tweak editor to only have h3-h6

See: https://github.com/GigaOM/gigaom/issues/5430 and https://github.com/GigaOM/gigaom/issues/5359
Perhaps add a `@todo` to filter, rather than replace this, so we don't stomp on what other plugins might add to the list?