feat: cleanup universities into an enum

TODO.md

flake.lock

flake.nix

server/launch_db.sh

...r/migrations/20241015182258_create_participants_table.sql

server/src/auth/claims.rs

server/src/bin/create_admin.rs

server/src/model/minimal_participant.rs

server/src/model/mod.rs

server/src/model/participant.rs

server/src/model/preview_participant.rs

server/src/model/staff.rs

server/src/model/university.rs

server/src/routes/get_participant.rs

server/src/routes/get_participants.rs

server/src/routes/mod.rs

server/src/routes/new_participant.rs