luci-base: fix placement of CBI strings

Commit c0de036b

 ("treewide: always include cbi.js") improperly removed the
cbi.js script include from header.htm, leaving behind the string dictionary.

Move the JSON dictionary to the parent <form> element and delete the
leftover </script> element.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>