Reorganizing the project's backend

[hanseaston]

Right now, the user and authentication part is handled by Google firebase/firestore, while all of the product's information is stored in MongoDB.

We could store everything in either firebase or MongoDB only. But part of this project's goal is also exposing newcomers to as many interesting tool stacks as possible, so that everyone has something they are interested to work on. Combining Firestore and MongoDB and let each one of them be in charge of one aspect of our shop seems like a fun and good way to go.

This structure needs to be further improved on. For example, for our MongoDB database, we can create one collection for each produce category, rather than storing everything in a gaint collection.

Please let me know what you think and how we can improve on the backend structure together. Questions, suggestions, comments are all welcome! ♥️

Listing this as one of the milestones because there will be consistent modifications to our backend structure as our project scales up.