Make Stripe optional

While Stripe is handy for the paid service, it might not be as useful when hosting your own instance (or when doing local development for that matter). This is an attempt to make the Stripe integration optional by disabling it if the Stripe keys are missing.

The lack of tests is something I intend to take care of, despite me getting distracted by #403. In the mean time, I would like to hear your feedback on the proposed functionality and the approach. Is this something that could be improved on?
This will read better with a block-if:

    if strip_enabled? && repo.private?
      raise ...
    end