V2 : fix loading of designs from remote URLs

[z3dev]

These changes fix the loading of designs from remote URLs, using the webserver as a proxy cache for the contents of the URLs. This allows:

• URL addresses containing hashes; http://jscad.xyz#http://external.com/data/design.js
• URL addresses containing queries; http://jscad.xyz#uri=http://external.com/data/design.js
• drag and drop of URLs

Testing was performed on MacOS and Windows 10, across the following browsers

• Safari
• Chrome
• Firefox
• Edge
• Opera

Note: The installation of the backend process is required to use this functionality. An additional note was added to the V2 Summary of Changes.

Fixes #688

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Does your submission pass tests?
  Thank you for your help in advance, much appreciated !