lml/ost#341: Add aggregated assignment exercise view.

WIP.

- Add permissions to assignment exercise model.
- Add links in the assignment page for the new view.
- Add assignment exercise controller with a single show action.
- Add assignment exercise view.

Notes:

- This commit introduces a lot of redundancy between student
  exercise and the assignment exercise.  Need to refactor to
  partials for better reuse.

- Assignment exercises are bunched for a particular cohort,
  need to make it accessible for all cohorts.

- The table border rendering is flaky in FF nightly.