Database Logic for Users

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Database Functions:

• [x] Create User (creates a new user in the Users table)
• [x] Set User Balance (sets the balance for an existing user in the Users table)
• [x] Get User Balance (gets the current balance of an existing user in the Users table)
• [x] Add to User Balance (adds an amount to a user balance)
• [x] Subtract from User Balance (subtracts an amount from a user balance)
• [x] Has enough Nav? (does User have at least the specified amount of NAVs?)