Add option to serve with URL prefix

[Vinno97]

Added the option to run Doccano with a URL prefix instead of always running from the domain root. The prefix is configured by specifying a BASE_URL environment variable. This PR fixes #1878

To run Doccano under /doccano/, set BASE_URL to /doccano/:

export BASE_URL=/doccano/

When we now compile the front-end, all routes are prefixed with BASE_URL.

We also need to let Django know about our new prefix. This is done using the same BASE_URL environment variable but needs to be set during runtime. I could not find a clean method to configure them both during runtime.

Since such a configuration will most likely only be used in combination with a load balancer, I also added the option to set USE_X_FORWARDED_HOST using environment variables.

Note that I did not write any tests for this (yet), both because I am unsure how this should be done and because I would first like to know if this PR would be acceptable.

[Hironsan]

Thanks!

Please write some test cases. After that, I'll merge this PR.

[Vinno97]

Great, will do. Any tips about where to start for someone unfamiliar with the project (and Django in general)? I expect I should extend backend/api/tests/test_config.py?

[lyingcrow]

Still in progress? @Vinno97

[Vinno97]

This hasn't been on my radar for quite a long time. The project I needed this fix for, was finished up a while ago. I'd be willing to write some tests for it, but I'm still equally unfamiliar with the project and Django. So some pointers will be needed