lml/ost#347: Integrate unconfirm gem.

- Add gem dependency.
- Add configuration file for unconfirm.
- Include JS and HTML.
- Add migrations.
- Modify free response and multiple choice submissions
  to use unconfirm.
lml/ost#347: Use git reference for unconfirm gem.
Merge pull request #352 from lakshmivyas/feature/unconfirm

lml/ost#347: Integrate unconfirm gem.