js: Switch to strict mode in script.js

The script should be compatible with strict mode, and strict mode
prevents some annoying bugs. The most important, it prevents assigning
to an undeclared variable.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

static/script.js

@@ -1,3 +1,5 @@
+"use strict";
+
 /* Tags menu filter */
 
 // Get a dictionary of tag name -> tag link from HTML