The course page's bottom bar on narrow screens: the course's name and size,
and the enroll action, kept at the bottom of the viewport while the page
scrolls.
On a phone the enroll card sits below the syllabus, so without this bar the
action would be a long scroll away. From lg the card stays beside the
sections and the bar is hidden. It sticks to its container's bottom edge,
so render it as the page's last child.
The course page's bottom bar on narrow screens: the course's name and size, and the enroll action, kept at the bottom of the viewport while the page scrolls.