add support for hiding under a subdir

[Jens Timmerman]

https://github.com/dani-garcia/vaultwarden/wiki/Hardening-Guide#hiding-under-a-subdir

[Jens Timmerman]

this is actually already possible by setting DOMAIN_PATH: "/vaultwarden" # results in a domain of https://example.com/vaultwarden/, needs to start with a '/' to a secret value