Support changing the baseURL for ProxySupport

[adamboutcher]

See issue - #228

This (probably terrible) change will allow you to use the env variable "MATHICS_DJANGO_URL" to support serving the files from a different BASE_URL.

This meant we could serve the application via a proxy, we were using "Open OnDemand" with a proxy configuration we couldn't change and also dynamic proxy addresses.

[adamboutcher]

The suggested change worked.

This all seems to work without applying a URL (or at least with my basic testing of pressing the examples button).

Only caveat is that the URL in the browser requires the trailing slash but that shouldn't be included in the URL variable.

[rocky]

@adamboutcher the changes are fine but would you please run

pip install black
black mathics_django/urls.py mathics_django/settings.py

Thanks.

[adamboutcher]

@adamboutcher the changes are fine but would you please run

pip install black
black mathics_django/urls.py mathics_django/settings.py

Thanks.

Done.