Refactor backend

[TiboX2021]

Refactor the backend & frontend structure to follow a schema that is more idiomatic to firebase.

• Refactor the seeder script to populate the database following the new schema
• Add firestore triggers to help populate the database and publish document data from private to public paths
• Update firestore rules to reflect the updated firestore structure
• Create data repositories for experiments, participants, chats, that expose a signal-based interface to firestore
• Create a global app state for the webapp that integrates all repositories
• Refactor the frontend to rely on the new appstate & repositories
• Refactor mutating cloud functions by direct firestore calls when possible, with data validation in the firestore rules

Leo's Edit: Still needed if @vivian proceed with a transition Angular -> Lit/MobX?
Thibaut's answer: to a certain extent, the Lit/MobX version will reuse the backend and the frontend repository structure, so this work is not all in vain.

• Refactor the frontend to reflect the cloud functions refactor changes)

[LeoLaugier]

@TiboX2021 just a reminder to add documentation in the readme to cp scripts/service-account.example.json scripts/service-account.json

[TiboX2021]

Ignore the base branch change, I wanted to force refresh the diff after merging #8, but it seems force pushing after a rebase does that already