security/vaultwarden: Make package rootdir friendly

• Use @dir with ownership instead of executing chown in postexec We should never chown -R the data directory at install (because it's empty) or when we upgrade (not needed)
• Fix the shipped configuration file, do not install it in etc/rc.conf.d, install it where vaultwarden will actually load it and use @sample

Differential Revision: https://reviews.freebsd.org/D41602