Fix key signature error page (#22229) (#22230)

- Backport of #22229
- When the GPG key contains an error, such as an invalid signature or an
email address that does not match the user.A page will be shown that
says you must provide a signature for the token.
- This page had two errors: one had the wrong translation key and the
other tried to use an undefined variable
[`.PaddedKeyID`](https://github.com/go-gitea/gitea/blob/e81ccc406bf723a5a58d685e7782f281736affd4/models/asymkey/gpg_key.go#L65-L72),
which is a function implemented on the `GPGKey` struct, given that we
don't have that, we use
[`KeyID`](https://github.com/go-gitea/gitea/blob/e81ccc406bf723a5a58d685e7782f281736affd4/routers/web/user/setting/keys.go#L102)
which is [the fingerprint of the
publickey](https://pkg.go.dev/golang.org/x/crypto/openpgp/packet#PublicKey.KeyIdString)
and is a valid way for opengpg to refer to a key.

models/asymkey/gpg_key.go

@@ -68,8 +68,16 @@ func (key *GPGKey) PaddedKeyID() string {
 	if len(key.KeyID) > 15 {
 		return key.KeyID
 	}
+	return PaddedKeyID(key.KeyID)
+}
+
+// PaddedKeyID show KeyID padded to 16 characters
+func PaddedKeyID(keyID string) string {
+	if len(keyID) > 15 {
+		return keyID
+	}
 	zeros := "0000000000000000"
-	return zeros[0:16-len(key.KeyID)] + key.KeyID
+	return zeros[0:16-len(keyID)] + keyID
 }
 
 // ListGPGKeys returns a list of public keys belongs to given user.