Create a Friends Expense

[ellaharrasser]

Create a Friends Expense between the current user and a friend.

• [x] An authenticated user is required for a successful response
• [x] New expense exists in the database after the request
• [x] Expense data returned includes the id, payer_id, receiver_id, description, amount, expense_date, settled, notes, created_at, and updated_at
• [x] Error response with status 400 is given when body validations for the description, amount, expense_date, and notes are violated