report a problem AJAX: deal with HTML coming back

currently, if the app returns status 503 on an AJAX (json) call,
nginx intercepts that and replaces the reply with HTML.

This change fixes the 'report-a-problem' javascript to handle this
situation.
move 'report-a-problem' JS into separate file
change $.ajax#complete to $.ajax#error, fix specs
Merge pull request #311 from alphagov/report-a-problem-ajax-deal-with-html-for-status-503

Report-a-problem: AJAX deal with html for status 503