Don't store XHR requests for user redirection after log-in.

In the case that users clicked 'Log in', but the last request the server
received was an XHR, they would be redirected to that XHR endpoint. This
was unhelpful.
Test the 'store_location' behaviour

'store_location' stores the current path in the session, to be used to
return the user to their last location after they've taken a diversion
to login. It is now disabled for Ajax requests.
Merge pull request #834 from moneyadviceservice/dont_redirect_to_xhr

Don't store XHR requests for user redirection after log-in.