Disable the Block Layout page for themes that are using Canvas if there is no block config

Overview

Currently if your theme is using Canvas for the layout, there is a warning message on the Block Layout page.

@lauriii said this was intentional so that sites that converted to Canvas could remove existing blocks, which makes sense. So if there is existing block layout config, we can keep it the same -- show the page with a warning.

Proposed resolution

If you are using Canvas and you don't have any block layout config, hide the page completely from the UI.

User interface changes

As described above :)