Add the ability to save user preferences

[deeheber]

Just favorite locations, but possibly more in the future.
Should limit to how many someone can add (maybe 5?). Ability to get, add, and delete locations.
Will also be nice to have auth for more robust security + being able to see how many users of the app there are.

Potential sub tasks to be created later

Infra to support the feature

• Add Cognito user pool to the cloud resources
• Integrate Cognito auth for the API
• Integrate Cognito auth for the frontend

The actual feature

• Add API endpoint to allow saving preferences in a DB (likely DynamoDB bc pricing + simplicity) > should limit how many favorites are allowed and allow show, add, delete operations
• Update UI to show, add, and delete favorites