Settings: remove SITE_URL, read ALLOWED_HOSTS from env
Settings: remove SITE_URL, read ALLOWED_HOSTS from env

1. `SITE_URL` is nowhere to be found in actual, official Django settings
   documentation, so it was removed.
2. `ALLOWED_HOSTS` now read data from environment, and use default sane
   value for AMY.

pbanaszkiewicz/amy,swcarpentry/amy,swcarpentry/amy,swcarpentry/amy,pbanaszkiewicz/amy,pbanaszkiewicz/amy