Fix targeted-feedback bug

Hey @sarina -- this fixes a bug in good old PR 1499 for the case where there are multiple targeted-feedback questions within one problem.

It's a small fix on the python side and some 100 lines of test. I've also tested it locally with a question from our DB class and it looks good.

Jira ticket:
https://edx-wiki.atlassian.net/browse/LMS-2667