js: use strict ... and fix the problems it points out

"use strict" gives stricter checks, both statically and at runtime. The
strictness tightens up the code and prevents some kinds of problems.

The <script> tag addition might not be pretty, but has consistently been added
with:

sed -i 's,<script>$,&'"'"'use strict'"'"';,g' `hg loc '*.html'`