explain mock identity provider behavior, update script

[plaurent]

Overview

This PR makes improvements to the Quickstart and to a related script to ensure that everything works for a new user interested in test-driving the Usher.

The current problem is that the server/scripts/get_idp_token_from_mockserver.sh script accesses the mock identity provider via host localhost:3002. Although an identity token is returned, the following steps won't work because (1) that host isn't whitelisted in The Usher's environment sample file, and (2) the identities in the test PostgreSQL database are not identified as authenticated using that host.

Summary of Proposed Changes

• The get_idp_token_from_mockserver.sh script was fixed to invoke the identity provider using idp.dmgt.com.mock.localhost:3002 as the host, which is a whitelisted host
• The Quickstart was updated to include a note to the effect that if the hostname idp.dmgt.com.mock.localhost is not recognized on the local network, it should be added to /etc/hosts.

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Open Questions or Items to Callout

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