lml/ost#340: Add exercise content(question) to the feedback page.

WIP.

- Add exercise content (question only) to the feedback page as a
  collapsible section.
- Remove trailing whitespaces.
- Remote trailing whitespaces in `show.html.erb` as well.

Notes:

* Free text/drawing provided by the student is not shown.
* The other options in multiple selection are not shown.
* The sidebar nav has misleading text ("Show" instead of "Go to")
* Collapsible sections could use simple CSS instead of inline JS.
* By introducing a section heading the header hierarchy is disrupted.