move bet validation to Bet data type

[ajlacey]

This PR is not ready. It:

• adds a Bets type to enable operating on multiple Bets at once
• moves ValidateBuy to models/bets
• updates function signature for ValdiateBuy to be a method on the Bet type, and to remove the Bet parameter from the signature

TODO:

• add test case for valid Bets
• add remaining test cases for invalid Bets
• add ability to use a test double in place of gorm db