WIP - Mixed content error for Issue #543

[jareddonovan]

Before your pull request is reviewed and merged, please ensure that:

• there are no linting errors -- npm run lint
• your code is in a uniquely-named feature branch and has been rebased on top of the latest master. If you're asked to make more changes, make sure you rebase onto master then too!
• your pull request is descriptively named and links to an issue number, i.e. Fixes #123

This is a work in progress for issue #543. There are two changes:

• Adding a config for Express to trust the proxy so that the https protocol is correctly used in the
  base tag.
• Fix the replacement of paths for links in scripts.

These probably aren't ready to merge, just posting so others can see my progress and put me on the right path :).

[catarak]

This seems to be working locally for me. I'm gonna merge this, and if it's not working in prod, revert it.

[catarak]

UPDATE: that didn't fix it 😄

[catarak]

I noticed that the base url is being generated differently server-side (https://github.com/processing/p5.js-web-editor/blob/master/server/controllers/embed.controller.js#L27), which would affect the /full/ view