Refactor Firestore Documents to Use Sub-collections

[scratchclaggy]

Context

Originally the user document contained a user's visits and contacts in an array. The intent was that this would reduce document reads (and therefore costs) for the firestore. This makes enforcing security rules for visits and contacts difficult as Firestore's security rules are not designed to work with large arrays.

Proposed Solution

Use sub-collections for the user document to store visits and contacts. This allows us to design security rules and data validation for each document in the sub-collection. Note that this may several refactors throughout the application to transition from the existing system.

Alternative solutions

There are two other ways to structure the firestore:

• Keep the current structure: this will make data validation difficult to implement, and may actually be impossible
• Use top level collections for visits and contacts: this would reduce document reads, but would increase complexity as the documents would each need to refer to their respective user.

Additional Notes

This blocks #145