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.

src/config.rs

@@ -19,7 +19,7 @@ static CONFIG_FILE: Lazy<String> = Lazy::new(|| {
 
 pub static CONFIG: Lazy<Config> = Lazy::new(|| {
     Config::load().unwrap_or_else(|e| {
-        println!("Error loading config:\n\t{e:?}\n");
+        println!("Error loading config:\n  {e:?}\n");
         exit(12)
     })
 });
@@ -872,6 +872,23 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
         err!("`EVENT_CLEANUP_SCHEDULE` is not a valid cron expression")
     }
 
+    if !cfg.disable_admin_token {
+        match cfg.admin_token.as_ref() {
+            Some(t) if t.starts_with("$argon2") => {
+                if let Err(e) = argon2::password_hash::PasswordHash::new(t) {
+                    err!(format!("The configured Argon2 PHC in `ADMIN_TOKEN` is invalid: '{e}'"))
+                }
+            }
+            Some(_) => {
+                println!(
+                    "[NOTICE] You are using a plain text `ADMIN_TOKEN` which is insecure.\n\
+                Please generate a secure Argon2 PHC string by using `vaultwarden hash` or `argon2`.\n\
+                See: https://github.com/dani-garcia/vaultwarden/wiki/Enabling-admin-page#secure-the-admin_token\n"
+                );
+            }
+            _ => {}
+        }
+    }
     Ok(())
 }