Anonymous feedback: use 'path' instead of 'url' everywhere

All anonymous feedback is now being created through the 'support-api'.
When persisting the feedback, the 'url' is no longer being set and this app
shouldn't be relying on it being set.

This fixes the issue with broken links in Feedex and missing URLs in the Feedex API.
Merge pull request #211 from alphagov/feedex-url-fixes

Anonymous feedback: use 'path' instead of 'url' everywhere