Displays worst bets for query
Adds 'is_best' checkbox to new bet form
Changes bet validation

* Changes bet 'is_best' default to false
* Disallows null, negative or 0 position for best bets
* Allows null or 0 position for worst bets
Show comments for bets on query page
Adds navigation from query show page to query index
Refactors tests to remove some duplication
Refactors bets controller

* Extracts method create params
* Memoize bet lookup
Adds AttributeStore and BetChangeNotifier classes

* Extracts common controller behaviour into Notifiable module
* Extracts construction of attributes for notification logic into
 AttributeStore value object
* Extracts notification logic into BetChangeNotifier service