fix: improve how we detect that the PR is managed by Mergify

The former implementation can detect any Mergify App icon in the page, even if the PR has been referenced by another one. We have to look for the "Summary" or the "Merge Protections" check-run.

Fixes MRGFY-4553

Change-Id: I2f812d20513947f8ba78f25c7d8fc46a27bef227

src/mergify.js

@@ -247,7 +247,7 @@ function tryInject() {
     }
 
     const appIconUrl = "https://avatars.githubusercontent.com/in/10562"
-    var isMergifyEnabledOnTheRepo = document.querySelector(`img[src^="${appIconUrl}?"]`)
+    var isMergifyEnabledOnTheRepo = document.querySelector(`img[src^="${appIconUrl}?"][alt="Summary"], img[src^="${appIconUrl}?"][alt="Mergify Merge Protections"]`)
     if (!isMergifyEnabledOnTheRepo) {
         return
     }