Student PR merge pipeline

[argvrutter]

On PR merge, execute scripts/org-membership.py

Proposed enhancements to scripts/org-membership.py:

• On duplicate detection: Assign duplicate tag to PR.
• On students not being invited: invite to team, add 'invited' label to relevant PR.

Modifications to students.yml will not be committed to main, but submitted via PR. If the PR already exists, then use that one.