[3] Write functions which will update currentUser in DB & state throughout App

[MitchellDill]

At the App.jsx level:

• Make POST to the server with the "currentUser" object that lives in state, to update the currentUser
• Create functions to pass down which will update Budget page and App-level currentUser with relevant information
• Create functions to pass down to dashboard page to handle transactions and updating App-level currentUser with relevant information
• Ensure all other pages are being passed all require information