[feat] configurable grading rubrics & calibration questions

[crystaltine]

Ref: HackGT/registration2#168 (comment)

API Side needs:

• new field on the Branch model to store a rubricJson and calibrationQuestionsJson
• Pull these dynamically in services/registration/src/routes/grading.ts so that we get the correct one for the current branch that's being graded
  • likely need a cache somewhere to avoid pulling each time
• if some branch doesn't have both a rubric and calibration questions, omit it from grading
• if no branch has those two setup yet, return a clean error to the frontend so Registration can display that