Feature/course banner
Changes the Banners that are displayed to the facilitators and maintainers of draft courses and cohorts. Now includes
- a checklist + links of stuff to do
- a publish button
- pretty pictures.
Also added checklist to the "complete a cohort" modal. Eventually, we will automatically move show this modal some time after the last event and move the manual trigger to complete a cohort to the settings page for the cohort (that page doesn't currently exist)