feat(security rules): implemented Firestore security rules

[ctrlc03]

Implemented security rules to protect data in the Firestore db. This includes test cases to verify the correctness of the security rules.

• users collection:
  • authenticated users can read and alter their own data (users collection)
  • coordinator can access everyone's data
• ceremonies collection:
  • any authenticated user can read details about a ceremony
  • coordinator can read and alter a ceremony collection

Useful read to understand Firestore rules.

fix #28