Implement user duo, initial version

TODO:
- At the moment each user needs to configure a DUO application and input the API keys, we need to check if multiple users can register with the same keys correctly and if so we could implement a global setting.
- Sometimes the Duo frame doesn't load correctly, but canceling, reloading the page and logging in again seems to fix it for me.

src/crypto.rs

@@ -19,6 +19,19 @@ pub fn verify_password_hash(secret: &[u8], salt: &[u8], previous: &[u8], iterati
     pbkdf2::verify(DIGEST_ALG, iterations, salt, secret, previous).is_ok()
 }
 
+//
+// HMAC
+//
+pub fn hmac_sign(key: &str, data:&str) -> String {
+    use data_encoding::HEXLOWER;
+    use ring::{digest, hmac};
+
+    let key = hmac::SigningKey::new(&digest::SHA1, key.as_bytes());
+    let signature = hmac::sign(&key, data.as_bytes());
+
+    HEXLOWER.encode(signature.as_ref())
+}
+
 //
 // Random values
 //