Allow flexible templates to define a region as the default/refuge region

[laryn]

Description of the need

Initially this thought came to me while working on Layout Paragraphs, but if it's possible to move the request upstream, I also think it makes sense for Flexible Layout templates in general. When creating a layout template, we should be able to define which region is the "refuge region" that orphan blocks flow into.

Proposed solution

I was thinking adding a toggle underneath each region:

[ ] This is the default region

Content that is not currently in a valid region (for example, after 
changing the current layout template) will flow into this region.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now allows users that create flexible layout templates in the UI to specify which region is the default (or refuge) region to use for orphaned blocks.