Weblate: Fix TLS configuration (sysadmin#17339)

  - Make Django interpret proxied requests as secure by setting
    SECURE_PROXY_SSL_HEADER.

  - Make Weblate generate URLs with https:// by setting ENABLE_HTTPS to
    True.

  - Make sure Django middleware will not attempt to redirect to HTTPS,
    as we do that in our proxy already.

  - Explicitely set CSRF_COOKIE_SECURE as True instead of make it depend
    on the value of another variable.

  - Add comments mentioning our proxied config for a couple of
    variables.