Implement mozreview button.

manifest.json

@@ -26,7 +26,7 @@
       "js": ["github-buttons.js"]
     },
     {
-      "matches": ["*://reviewboard.mozilla.org/*"],
+      "matches": ["*://reviewboard.mozilla.org/r/*"],
       "js": ["mozreview-buttons.js"]
     },
     {

mozreview-buttons.js

@@ -0,0 +1,27 @@
+const revision = document.querySelector('#review-request-inputs').firstElementChild.value.match(/[0-9a-f]+/)[0];
+
+// Create 'build in janitor' link.
+const a = document.createElement('a');
+a.href = '#';
+a.target = '_blank';
+a.title = 'Build patches in Janitor';
+a.textContent = 'Janitor ▾';
+a.style.color = 'white';
+
+const li = document.createElement('li');
+li.classList.add('has-menu');
+li.style.backgroundColor = '#007EC6';
+li.appendChild(a);
+
+const menu = document.createElement('ul');
+menu.classList.add('menu');
+menu.style.display = 'none';
+menu.innerHTML =
+  '<li><a href="https://janitor.technology/create/?project=firefox-hg&revision=' +
+  revision + '" target="_blank" title="Build patches on Ubuntu 16.04">Build on Linux</a></li>' +
+  '<li><a href="https://janitor.technology/create/?project=firefox-windows&revision=' +
+  revision + '" target="_blank" title="Build patches on Windows 10">Build on Windows</a></li>';
+li.appendChild(menu);
+
+const buttons = document.querySelector('.actions .actions-right');
+buttons.insertBefore(li, buttons.querySelector('.primary'));