UX: Show edit button on post revision modal as long as user can edit.

test/javascripts/controllers/history-test.js.es6

@@ -0,0 +1,28 @@
+moduleFor("controller:history");
+
+test("displayEdit", function() {
+  const HistoryController = this.subject();
+
+  HistoryController.setProperties({
+    model: { last_revision: 3, current_revision: 3, can_edit: false }
+  });
+
+  equal(
+    HistoryController.get("displayEdit"), false,
+    "it should not display edit button when user cannot edit the post"
+  );
+
+  HistoryController.set("model.can_edit", true);
+
+  equal(
+    HistoryController.get("displayEdit"), true,
+    "it should display edit button when user can edit the post"
+  );
+
+  HistoryController.set("model.current_revision", 2);
+
+  equal(
+    HistoryController.get("displayEdit"), false,
+    "it should only display the edit button on the latest revision"
+  );
+});