Allow `SECRET_KEY` and `LDAP.BIND_PW` to be read from file

Allow the sensitive SECRET_KEY to be imported using a file containing the secret. For example adding SECRET_KEY_FILE.

Reasoning: Putting secrets directly into configuration files is a bad pattern. Further, it creates a secret which is fragile for automation, since it requires you to modify the toml file directly with for example Ansible's lineinfile.