Apply image browser redesign to slide selection for courses

[ingridtv]

Adds the filtering options for the slides, and adds styling (green box + check mark) to slides that have been added to the course.

At a later stage we might want to

• make the page not reload every time filtering/add/remove is performed,
• add pagination to avoid loading all slides simulataneously, and
• list the added slides in a list/side panel/overlay that is always visible on the page,

but for now I think this is an improvement