[Pending review] Payment bugs

Merge PR #188 before this!

Bug fixes listed in Trello card:

I tested payments in braintree.sharetribe.us and noticed following bugs/open questions:

* Remove `:sandbox` from BraintreeService.configure method. Move it either to environment configurataions (`:production` in production, `:sandbox` in testing) OR move it to the `communities_payment_gateways` (preferred?)
* When a new payment is created by the invoice form its type is always CheckoutPayment (the payment `type` column) even though it should be BraintreePayment
* Logging in BraintreeWebhooksController is broken. Use `Rails.logger.info` etc
* I got `AuhtorizedError` and ugly error page when payer created a listing but didn't have BraintreeAccount and a buyer tried to pay it. (Expected behaviour: Payer should not be able to accept request without payment details)
* Commission in invoice form seems to be whole number without decimals and thus it differs from the actual service fee (see screenshot)
* In listing page we there is a text "A 12% service fee wil be added to the price". Expected: "A 12% service fee will be reduced from the price"