feat: refactor user model, and add institutions attribute

[lchen-2101]

closes #26

[github-actions]

Coverage report

The coverage rate went from 82.21% to 82.73% ⬆️
The branch rate is 84%.

100% of new lines are covered.

Diff Coverage details (click to unfold)

src/entities/models/init.py

100% of new lines are covered (100% of the complete file).

src/entities/models/dto.py

100% of new lines are covered (100% of the complete file).

src/oauth2/init.py

100% of new lines are covered (100% of the complete file).

src/oauth2/oauth2_backend.py

100% of new lines are covered (41.17% of the complete file).

src/routers/admin.py

100% of new lines are covered (92.59% of the complete file).

[hkeeler]

Overall looks good. Just the one question about parse_institutions.