Make keypair configurable

[thekaveman]

As part of moving this app towards production-ready, we need to be able to configure the keypair used as part of the Eligibility Verification token work.

We currently hardcode paths to the sample keys:

with open("./keys/server.key", "rb") as pemfile:
    server_private_key = jwk.JWK.from_pem(pemfile.read())
with open("./keys/client.pub", "rb") as pemfile:
    client_public_key = jwk.JWK.from_pem(pemfile.read())

Let's move these into a new couple of settings variables to allow them being overridden.