Save gist ID in config.json in customizer

[pdobrigkeit]

In addition to #11004 I want to suggest that the generated gist ID is saved inside the config.json, as that offers a very easy way to "import" old configurations. One just loads the customizer with the associated gist ID http://getbootstrap.com/customize/?id={yourId} The main problem is right now that once you close the browser you have no idea which ID you had given.

I will be working on a PR for this on the weekend.