nixos/gitlab: Add support for storing secrets in files

Add support for storing secrets in files outside the nix store, since
files in the nix store are world-readable and secrets therefore can't
be stored safely there.

The old string options are kept, since they can potentially be handy
for testing purposes, but their descriptions now state that they
shouldn't be used in production. The manual section is updated to use
the file options rather than the string options and the tests now test
both.