Teacher check in improvements

This adds class flags and detail about the class to teacher check-in.  It's hidden by default so it doesn't take up much space.  I refactored the backend part of the view a bit to make this easier, and maybe a bit faster.

This is built on #1234 (so it's really just the last 4 non-merge commits Github shows, see diff [here](https://github.com/learning-unlimited/ESP-Website/compare/class-flags...teacher-check-in)) and resolves #489, the rest of #499, and #1240.
I didn't even notice this over on the side.  And, as a non-admin, I probably wouldn't realize that the titles were clickable. Is there any way to style the class titles to make this more obvious? If not, can you add "(click for more info and class flags)" after each class title?