Disable sign up to notifs for sections in past semesters

[mehallhm]

Blocked by 'Delete notifications from past terms' - https://github.com/orgs/sandboxnu/projects/20/views/1?pane=issue&itemId=80297375

Problem: It doesn't make sense to allow users to sign up for notifications for past semesters: users no longer need to get into those sections. Our "signed up" state is also not super clear.

Solution: Remove ability to sign up for SMS messages for semesters that have been marked as "inactive".
(remove button)