Add check if people are still studying

[T8902]

Closes #ISSUE.

Summary

How to test

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'

[sentry-io[bot]]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: website/members/models/membership.py

Function	Unhandled Issue
clean	ValueError: 'User' instance needs to have a primary key value before this relationship can be used. ... Event Count: 1

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