Assure that endpoints are up to date with IdP

[jgrevich]

Issue

As a login.gov load tester,
I would like to make sure the fake server's endpoints are up-to-date with the IdP,
So that I can load test the IAL2 flows that this codebase mocks.

Solution

• migrate Accuant endpoints to LN TrueID for doc auth
• add USPS IPP flows (Stub USPS routes with fixture responses #24)
• add any other diffs between current IdP proofing endpoints and this repo to the issue

Acceptance Criteria

• sp_ial2 load tests complete without failure using this mock proofer.

[timothy-spencer]

A nice outcome from this issue would be that we know how to configure the idp so that it uses https://github.com/18F/identity-fake-server , which is deployed at https://fake-server.loadtest.identitysandbox.gov/