Allow choosing the new version of themes in the design sidebar

To safely migrate production content to the new version of themes, we can add the new versions as an option while keeping the old versions in case a particular page has issues.

This migration will be handled by the Planet 4 team, which is manageable given the amount of content.

For a short while this means that both the old and new version will coexist in the dropdown. The current PR for this adds "(old)" and "(new)" to each to avoid confusion. Once all is switched these extra options are removed again and it will only list the new versions.

(Don't mind the red message on saving in the attached video, it's a minor issue with the page data we still need to solve but doesn't break anything)

Currently deployed on develop sites, most pages are on GPI.