Added support for Argon2 hashing support for the `ADMIN_TOKEN` instead
of only supporting a plain text string.
The hash must be a PHC string which can be generated via the `argon2`
CLI **or** via the also built-in hash command in Vaultwarden.
You can simply run `vaultwarden hash` to generate a hash based upon a
password the user provides them self.
Added a warning during startup and within the admin settings panel is
the `ADMIN_TOKEN` is not an Argon2 hash.
Within the admin environment a user can ignore that warning and it will
not be shown for at least 30 days. After that the warning will appear
again unless the `ADMIN_TOKEN` has be converted to an Argon2 hash.
I have also tested this on my RaspberryPi 2b and there the `Bitwarden`
preset takes almost 4.5 seconds to generate/verify the Argon2 hash.
Using the `OWASP` preset it is below 1 second, which I think should be
fine for low-graded hardware. If it is needed people could use lower
memory settings, but in those cases I even doubt Vaultwarden it self
would run. They can always use the `argon2` CLI and generate a faster hash.