idea(webdav-utils): in place upgrader

[saqimtiaz]

See TiddlyWiki/TiddlyWiki5#7928 for an ideal scenario. Alternatively, we can mirror the upgrade.html locally and inject the needed code into it and use it. Would need to cache it locally perhaps using the version number in the file name so we don't fetch it unnecessarily given the size.

An alternative to postMessage would be a paramifier based approach of opening upgrade.html locally in an iframe with the correct URL for the wiki to upgrade in a part of the URL hash. That wiki gets imported and upgraded we override save to save back to the original path of the wiki that was upgraded.