Admin token Argon2 hashing support

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.

Cargo.toml

@@ -157,8 +157,19 @@ semver = "1.0.16"
 mimalloc = { version = "0.1.34", features = ["secure"], default-features = false, optional = true }
 which = "4.4.0"
 
+# Argon2 library with support for the PHC format
+argon2 = "0.5.0-pre.0"
+
+# Reading a password from the cli for generating the Argon2id ADMIN_TOKEN
+rpassword = "7.2"
+
 # Strip debuginfo from the release builds
 # Also enable thin LTO for some optimizations
 [profile.release]
 strip = "debuginfo"
 lto = "thin"
+
+# Always build argon2 using opt-level 3
+# This is a huge speed improvement during testing
+[profile.dev.package.argon2]
+opt-level = 3