Splitting bet expiration date & time into two columns

Along the way, this PR made the following changes:

* using poltergeist to run headless browser specs with JS
* adding `Bet#expiration` virtual attribute that returns a `DateTime` object by combining `expiration_date` and `expiration_time` attributes for a bet instance
* using `pickadate.js` library for better date & time selection UI
Line is too long. [118/80]