Refactor eligibility verification into reusable package

[thekaveman]

This is a corollary to cal-itp/benefits#141, with which this work must coordinate. See that issue for the general overview.

Server specifics

eligibility_server/verify.py is where the server-side implementation lives.

Some of the code there is setting up the flask_restful resource and dealing with HTTP request processing, and that would stay. Anything concerned with creating and processing the Eligibility Request and Response objects could be moved into the new package:

• _get_token_payload()
• _make_token()
• Code that crafts the response payload

[angela-tran]

I did some analysis on how easily we can replace eligibility-server code with calls to eligibility-api, given all the development that's happened in server since cal-itp/benefits#141 was done.

_get_token_payload and _make_token should be easy to do. It's almost a direct replacement - here's a diff of those two functions in eligibility-api vs. eligibility-server.

eligibility-api does not currently have logic for crafting the response payload, so we'll need to figure out what that function should look like.