Fix unescaped curly braces in javascript formatting

This would previously throw an error due to `format` interpreting the
curly braces as placeholders

backend/injector.py

@@ -261,17 +261,18 @@ class Tab:
 
             wrappedjs = (
                 """
-            function scriptFunc() {
+            function scriptFunc() {{
                 {js}
-            }
+            }}
-            if (document.readyState === 'loading') {
+            if (document.readyState === 'loading') {{
-                addEventListener('DOMContentLoaded', () => {
+                addEventListener('DOMContentLoaded', () => {{
                 scriptFunc();
-            });
+            }});
-            } else {
+            }} else {{
                 scriptFunc();
-            }
+            }}
             """.format(
+                    js=js
                 )
                 if add_dom_wrapper
                 else js