Delete notifications from past terms

[mehallhm]

THIS DESCRIPTION OUT OF DATE PLS PING ME

Problem: It doesn't make sense to keep notifications from past semesters: users no longer need to get into those sections

Solution:

Every time the updater runs, we should delete all notifications from term IDs marked as inactive.

As a secondary issue — we need a way to determine which semesters are active, and which ones aren't.

Depends on : https://trello.com/c/CQ0p1Ms4/391-delete-notifications-from-past-terms

Resident Expert: @zacharh