🔧(backend) support _FILE for secret environment variables

Allow configuration variables that handles secrets, like
`DJANGO_SECRET_KEY` to be able to read from a file which is given
through an environment file.

For example, if `DJANGO_SECRET_KEY_FILE` is set to
`/var/lib/meet/django-secret-key`, the value of `DJANGO_SECRET_KEY` will
be the content of `/var/lib/meet/django-secret-key`.

CHANGELOG.md

@@ -8,3 +8,5 @@ and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
+- 🔧(backend) support `_FILE` for secret environment variables #566