fix(url): define server name to return full urls

It's using `SUPERDESK_URL` environment variable which is parsed
and used for `URL_PROTOCOL`, `SERVER_NAME` and `URL_PREFIX`.
Revert "fix(upload): generate non-external urls to make it work with proxy"

This reverts commit 5f8f133a6f4dcc38e0e914458894d3bb363d2157.
fix(settings): remove default URL_PREFIX

there is no prefix by default, only when set