Feature/exercises

WIP. Refactored and consolidated the previous pull requests.

- [X] Pull all current work into a single pull request.
- [X] Reuse the same exercise partial for both 340 and 341.
- [X] Disable whitespace clean-up for OST project.
- [X] Make question partial take `collapsability` params (solved with a partial layout instead.)
- [X] Ensure that rbenv.md says rbenv is optional
- [  ] Show free responses in the feedback page.
- [  ] Fix styles for multiple choice display.
- [  ] Enable researchers view in the assignment exercises page.
- [  ] Add breadcrumbs / navi for assignment exercises page
- [  ] Sort and group students in the assignment exercises page.
- [  ] Get assignments under multiple cohorts working.