Add scroll-to-top when switching verticals from bottom nav buttons.

Bug: When a student finishes a tall vertical and clicks the next arrow at the bottom of the page, the vertical advances but the students view remains at the bottom of the page.

This adds a scroll to top when the students clicks the nav buttons at the bottom of the page.
Also, a small refactor so that the buttons call the same method with an argument of `seq_next` or `seq_prev`.

Reviewers:
@rlucioni 
@jkarni