Add an option to enable access via different URL

[izzy]

Feature request:

Current behavior: Currently the server is only accessible via 'proto://server/'.

Expected behavior: It would be good to have a configuration option to set a different URL. This would make integrating the DocumentServer in different environments possible. One could use the DocumentServer behind proxies/load balancers and in other apps.

The configuration might look like the following:

{
    'protocol': 'https',
    'server': 'example.com',
    'port': '8443',
    'path': '/subdirectory/'
}

This would force the server backend and server frontend to use https://example.com:8443/subdirectory/ as base-URL.

The only reference to the actual URL we found digging through the source was in sockjs around line 5343, but this only works for the frontend part. For the backend we did find requests for cached files in /cache, but could not find the actual backend source where the URL is generated.