Replace Google Sheets with Firestore

[miike]

The current application uses Google Sheets as the primary datastore. Although flexible and easy to edit it has a number of disadvantages including:

• Difficult to achieve real time sync (doable through Google Apps Script but not ideal)
• More difficult to insert data that has been generated programatically (e.g., through reading Aruco markers). Having an API or database to interact with directly would make this significantly easier.

Firestore allows us to setup a minimal implementation of real time sync between the database and the UI. Although Google offer a basic interface for editing documents within Firestore we'd likely need to build an additional interface which does not currently exist for manually editing talks.