Remove course name from page H1, and slightly improve sequence navigation

This is nominally for LMS-1923 (and others) but there already was an H1 on the page. These are just minor tweaks.

To me, the header in navigation.html looks fine, except that the H1 contains an image. But since the image has an alt attribute, screen readers should have no problem knowing what's going on.

Thoughts, @frrrances ?