created RPC for querying and storing user reports

[ethannchen]

1. created UserReport schema
   • current schema is implemented according to doc: [1thing tracker self-report design - Google Docs]
   • final schema will need Li’s verification ⚠️
2. created RPC
   • created API for querying user reports from Firebase
   • created API for storing unverified report records in Firebase
3. deployed to App Engine (dev)
4. tested against remote server (PASSED)
   • sample request (get user reports):
   • sample request (post a user report):
5. created a new branch "self-report-dev" (based on "notif-gae-dev") on main repository, and created new PR to "self-report-dev" branch