Support for running behind a reverse proxy

[allo-]

I tried to run spotmop behind a reverse proxy. This seems quite hard to accomplish. What needs to be done:

• URLs need to be relative to the current url. Often they are absolute to the server. (Workaround: Use it on a subdomain instead of subfolder)
• The Port of the server should be used (works mostly, seems to have problems with websockets)
• HTTPs URLs need to be used if spotmop is requested using https.

But the biggest problem I saw is requesting mopidy at 6681 without a way to configure urls or use rewriting. Either it should request localhost:6681 on serverside or allow to configure a url which can be a reverse proxy to server:6681. Just requesting with the port isn't feasible for such a web-setup, which should NOT expose the server directly.