Add endpoints for getting exams from the exam bank

[EarthenSky]

Close #56

TODOs

• implement tables to store exam metadata
• create alembic migrations
• test that cas auth works correctly w/ the frontend
• test that sqlalchemy join works as expected
• finish the metadata endpoint
• test that the watermark functions work correctly
• finish the exam get endpoint
• go through all exams & store data in a spreadsheet, to load into the db
• deploy the changes

Later

• add an endpoint for professors (faculty) to mark an exam as not theirs, or to mark an unknown exam as theirs