Toggle button to hide all comments while showing code for a walkthrough

[blerner]

One useful scenario for the Submissions#details endpoint is to conduct a codewalk with students. in that setting, it would be nice to easily hide all comments (style, manual grading, whatever) so that only the pristine code is shown without any distractions or prejudicial detractors. Add a toggle button to easily hide all comments (like how is done to hide style comments while manually grading).