Gutenberg: Add option to remove h1 from heading block

[myleshyson]

This is currently possible with the classic editor through the tiny_mce_before_init filter.

I know a decision was made to deemphasize H1 which was great. However it would be even better if there was a way to remove it entirely for businesses, agencies, universities, etc that use H1 in the actual template so that admins can't accidentally (or purposefully) mess up seo for the larger organization like ours.

Anywhere that this could be added would be great. Maybe a filter similar to tiny_mce_before_init.

Thanks

[swissspidy]

Previously: #13565

[myleshyson]

Yea but this is a different matter. Our organization actually just wants a way to completely remove it. #13565 wanted to add it to the title bar.

[gustavpursche]

Please not only h1, but any heading. For many cases it doesn't make sense allowing six levels of headings.

Also, I really like the google docs UI, where you can only select an h3, for example, if an h2 was used previously. Prevents many errors, where editors are just using a headline level because it looks the way they need it.

[jakewhiteley]

I also need this.

A lot of agencies use WordPress for client websites, and it is likely the case that most templates already have a H1, or as @gustavpursche pointed out, the theme we provide them does not have styling for a H6 for example.

We need a way to restrict what clients can add - for their own good!