Add google maps as an embed

[karmatosed]

This is a user request from the forums: https://wordpress.org/support/topic/great-new-tool/#post-9765086

[overclokk]

Maybe this is better in plugins and not in core bacause gmaps has a lot of customizations.

[aduth]

Related: #315

[ellatrix]

We can do this without API keys or oEmbed. #4660 semi addresses this, but in addition to that we could create a proper embed block for Google maps where the user can paste the iframe embed code and the block just saves the iframe attributes and URL. A content filter could put this into an iframe similar to the way oEmbed works, so in other words, the Google Maps domain would be whitelisted to use iframes.

[ellatrix]

Anything other than the iframe embed code would be plugin material though.

[mtias]

Duplicate of #315.