UX: Allow saving with cmd+s/ctrl+s with Ace editor (#12674)

When editing the files for a theme in the admin dashboard, typing "cmd+s" (a common key-binding to save in most text editors) used to engage the browser's default "save page" dialogue.

This commit adds a key-binding to the ace editor that saves the file.

Now, the "cmd+s" (and "ctrl+s" for windows) key-binding does the same action as the save button.

app/assets/javascripts/admin/addon/components/admin-theme-editor.js

@@ -120,5 +120,9 @@ export default Component.extend({
     onlyOverriddenChanged(value) {
       this.onlyOverriddenChanged(value);
     },
+
+    save() {
+      this.attrs.save();
+    },
   },
 });