Add timestamp to debuginfo footer text.
Get rid of awkward and underused utils.to_js.

Buttonlite didn't really do anything with this function since it
disabled callback *and* escape.

The response wrapper fixes a bug where the callback is a unicode object
and since the content object is UTF-8 encoded things go south ("'ascii'
codec can't decode byte ...").

This also removes a few scattered imports of the function that were
unused.
embed/wired/widget: Add callback validation à la JSONP.
ErrorController: Don't ever return a body in 204/304 responses.

This should fix an issue where we return a JSON body for 304 responses
to API requests which is invalid according to the HTTP spec and appears
to be causing bad data to get cached in the CDN.
ErrorController: Fix bad failien URL on handle_awful_failure case.
Fix use of StackedObjectProxy pylons.response in debug error pages.

This caused some debug page static files to generate an error.