Switch to Builder Template without losing content

[jeremyfelt]

It's currently possible to switch from the Builder Template to another template without losing data as the primary post_content for that page populates as expected.

When going in reverse, the Builder Template is not aware of post_content attached to the page as the admin view is trying to view the pieces of the content through post meta so that it can build them into a single piece of content.

We should have a way to save the data so that somebody can switch from another template to the builder template and automatically populate a single section with existing content. This should make importing existing sites a much more pleasant experience.