Render Layout Builder block edit forms in an iframe, using the admin theme.

Motivation

Layout Builder uses off-canvas for every section or block form, while also rendering the entire layout page in the frontend theme. This approach works for simple forms with a few text fields, where styling is easy. But when using more complex form elements like tabs, accordion, entity autocomplete, media library or entity browser, the amount of work required to get everything to look (and work) good can be huge.