Moved constants to a single place
- moved constants/string literals used for request status and route names to a separate file const.py. Refer them with RequestStatus.* or RouteName.* to prevent typo/inconsistency.
 - cleaned up urls.py
 - cleaned up challenge.py to replace the route name literals with calls to uri_for/redirect_to
 - removed the Requests page, as it is no longer being used