MM-21103: change plugin signature path (#13360)

* MM-21103: change plugin signature path

Save as `<plugin_id>.tar.gz.sig` instead of `<plugin_id>.sig`. The latter was a relic of the previous design to support multiple plugin signatures, but now creates an inconsistency with how the original source files were supplied as `<some_name>.tar.gz` and `<some_name>.tar.gz.sig`.

Fixes: https://mattermost.atlassian.net/browse/MM-21103

* relax signature matches to avoid assuming signatures always exist

app/plugin_signature.go

@@ -119,6 +119,7 @@ func verifyBinarySignature(publicKey, signedFile, signature io.Reader) error {
 	}
 	return nil
 }
+
 func decodeIfArmored(reader io.Reader) (io.Reader, error) {
 	readBytes, err := ioutil.ReadAll(reader)
 	if err != nil {