fix(settings): ensure environment variables always override config

[Xiretza]

DJANGO_STATIC_ROOT is required in order to run collectstatic in a packaging environment - manually specifying it as an environment variable should always override any config files that may exist.

This will break setups where DJANGO_STATIC_ROOT/DJANGO_MEDIA_ROOT are set to some incorrect value in the environment; is that a realistic concern?

[daftaupe]

@tasn I think these changes could be merged and we could roll a new release, what do you think ? Last release is really old and at least https://github.com/etesync/server/issues/178 shows that people tend to stick to latest release available.