Revert "Merge pull request #395 from sharetribe/disable-contact-request-notifications"

This reverts commit 201fc73e2b944bff4abc789e4e571738439fe69e, reversing
changes made to 967d7f0f7b088b08ef4ed513344d21c996627293.
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Merge pull request #398 from sharetribe/contact-request-notifications

Contact request notifications
Cleaner payment classes
Move methods to CheckoutPayment