Check we have a proper url.

Will not allow injection attacks
Merge branch 'master' of github.com:guardian/frontend into id-bugs
That is a bit cleaner than the try{}catch{}
Merge branch 'master' of github.com:guardian/frontend into id-bugs
Cleaned up domain verification
Merge pull request #5702 from guardian/id-bugs

Check we have a proper url for the value "returnUrl".