Track decryption success/failure rate with piwik

Emit a piwik event when a decryption occurs in the category "E2E" with
the action "Decryption result" and the name either "failure" or
"success".

NB: This will cause Riot to a lot of networking when decrypting many
events. One HTTP request per decrypted event should be expected.

src/components/structures/MatrixChat.js

@@ -1308,6 +1308,15 @@ export default React.createClass({
             }
         });
 
+        // XXX: This will do a HTTP request for each Event.decrypted event
+        cli.on("Event.decrypted", (e) => {
+            if (e.isDecryptionFailure()) {
+                Analytics.trackEvent('E2E', 'Decryption result', 'failure');
+            } else {
+                Analytics.trackEvent('E2E', 'Decryption result', 'success');
+            }
+        });
+
         const krh = new KeyRequestHandler(cli);
         cli.on("crypto.roomKeyRequest", (req) => {
             krh.handleKeyRequest(req);