Use and enforce "use strict"; for JS

.eslintrc

@@ -45,5 +45,5 @@ rules:
     max-statements: [2, 30]
     no-bitwise: 2
     quote-props: [2, "always"]
-    global-strict: 0
+    strict: ["error", "global"]
     quotes: 0

qutebrowser/javascript/scroll.js

@@ -17,6 +17,8 @@
  * along with qutebrowser.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+"use strict";
+
 function _qutebrowser_scroll_to_perc(x, y) {
     var elem = document.documentElement;
     var x_px = window.scrollX;

qutebrowser/javascript/webelem.js

@@ -17,6 +17,7 @@
  * along with qutebrowser.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+"use strict";
 
 document._qutebrowser_elements = [];