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.

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

```sh
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, but needs to be set during runtime. I could not find
a clean method to do 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 these URLs should probably not be defined in the view anyways.
I patched them here because I am not familiar enough with the project to
fix this cleanly.