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.
Oh, I see, you also add phone numbers here.

Unfortunately, this doesn't work.  section.teachers is a property, not a list, so you're not modifying anything here.