fix challenge complete email logic, and notifying users when challenge / task cannot be found
do not abort with a 400 bad request when the challenge exists

Instead, we should be updating the challenge. to be implemented. for
now, return empty message.
adding update cascade to foreign keys
Merge branch 'develop' of github.com:osmlab/maproulette into develop

* 'develop' of github.com:osmlab/maproulette:
  Fixing typo in function ChallengeComplete function

Conflicts:
	maproulette/static/js/maproulette.js