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
> WordPress doesn't set a default

But another plugin might set a value there, and we'd certainly want other plugins to filter rather than replace for compatibility with this.