Fundraising tech/New integration manual

This explains the tech team process around creating a new integration.

Preliminaries

 * Review documentation and confirm that the payment processor seems to comply with our technical, security, and communication standards.
 * Create test accounts.

First milestone: ready for internal test
When this milestone is completed, we will be able to deploy code to staging or production which demonstrates some basic payment workflow. Fundraising and tech team members will be able to donate. We should not expose the page to the public at this point.

What we need for internal testing: What we can skip for this step:
 * Working frontend form.
 * Backend processing workflow is able to make a charge and push the message onto the queue.
 * Audit processing. We can always pick these files up and import in the future.
 * Listener. Some workflows rely on the listener to finalize the payment, in this case we usually want to include that in the 1-hour test.
 * Not strictly necessary that we can import the queue message, this is sort of a grey zone.

Second milestone: 1-hour donor test
We will have a frontend stable enough to:
 * Provide decent donor user experience.
 * Process audit files and potentially listener messages, in order to send the donors their thank-you letters in a timely fashion. Ideally, this is a 100% thing, otherwise Tech and Donor Services will have to play catch-up.

Final milestone: campaign-ready
We have iterated with 1-hour tests until we're certain that the bugs are ironed out.