fix: allow course staff role to be updated with email change

[alangsto]

JIRA: COSMO-478

Description:
If a user is added to a course with either staff or instructor permissions, changes their email, and then is added to another course as staff or instructor, the second event meant to sync staff role permissions with edx-exams will fail due to an integrity/unique constraint error. This is because the call to get or create a new User object in the event handler is keyed off of both the username and email fields, which means that subsequent email changes will result in calls to create a new User object with the same username but new email, as opposed to just getting the existing User object.

This bug is fixed by only using the username field to get or create a new User object.

Merge checklist:

• All reviewers approved
• CI build is green
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Delete working branch (if not needed anymore)

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
edx_exams/apps/core/tests
test_handlers.py
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