Cache selected teachers/courses when filtering changes

Caching the list of teachers is simple, as it's a string of numbers and commas. The problem is that when using a white list, no new teachers are shown. The solution would be a black list.

White list:

• [+] Only the selected teachers are shown
• [-] New teachers are hidden

Black list:

• [-] Not only the selected teachers are shown
• [+] New teachers are shown