https://service.deployment.vendor.tld/service-base/api-prefix/hard/path
|___________________________________| |__________| |________| |_______|
             BASE_HOST                  BASE_PATH    PREFIX    Hardcoded
|_________________________________________________|
             SERVICE_BASE_URL

- Introduce <SERVICE>_BASE_URL setting, and <API>_PREFIX constants
  for all services. Extract BASE_HOST, BASE_PATH from the setting.

- Use settings and constants to construct top-level patterns
  (base-path/api-prefix) in services' urls.py

- Refactor/extend proxy() to be compatible with the above.

- Change URLs passed around (ui, e-mails) to be compatible.
  (needs more work)

Notes:

This configuration REQUIRES that any rewriting and proxying on the
front-end must eventually result in the application server (gunicorn)
getting the original path of the request, otherwise the view-generated
URLs will be different.

Changes:
- set top-level urlpatterns from <SERVICE>_BASE_URL
- rename setting: APP_INSTALL_URL to CYCLADES_BASE_URL
- rename setting: ASTAKOS_URL to ASTAKOS_BASE_URL everywhere
- introduce setting: PITHOS_BASE_URL
- update shipped .confs, quick install admin guide, upgrade guide
- fix urls/views/reverse()/other that were broken
- (more not listed)