Figure out appropriate handling for case where a class gets deleted

[patudom]

For students who were in a class and then the class gets deleted, the student should continue to exist (with their "classmates" known to them in their story state), but their class summary will no longer exist, which may cause problems.

(We could possibly test this by creating a class, filling it with fake students, deleting the class, then logging back in with a student and see what breaks and safeguard against those issues).

Whatever the issue with the student experience, we should check that if a student is in a class that got deleted, that their measurements and age info don't end up in the "ALL_STUDENTS" summary table.