Course staff page double-renders sometimes

[nwalters512]

Similar to the issue described in #275, the course staff page double-renders sometime. We should apply the same fixes there - rewrite with hooks and track loading state locally. This is also a good opportunity to bring that page in line with the visual style of the rest of the "settings" pages across the site. We can change the page into "course settings" instead of limiting it to queue staff so that it can serve as a home for any future course-wide settings (webhooks come to mind).