Add Developer Mode for Freshbooks

[b-f-chan]

Add dev mode to billing app to mockup connection to Freshbooks

[justincorrigible]

Must add endpoint to generate tokens for new devs:
(from freshbooks docs - https://www.freshbooks.com/api/start)

"The redirect URI should be an endpoint in your application that will receive the authorization code and will use it to retrieve the bearer token and refresh token. These will be necessary to make authenticated calls to the FreshBooks API."