Change the functionality of the grader sidenav

[yammesicka]

Currently, we share the most popular sidenav comments.

After using the system for a while, I think the best behavior should be:

1. Allow adding custom comments.
2. Remove the automatic feature of showing the popular ones automatically.
3. Check how to save page real-estate and organize the comments properly.
4. Have a small expansion button for the popular comments (yours, and that the rest of the group submitted). Show numbers near each one. Allow the user to add these comments to his favorite ones with a click.