Summary:
Adds a fun new UI for adding accounts to the sync engine. After creating your sync engine instance, all you have to do is auth your accounts on the command line and then enter the URL/port number in this flow. That pulls all of your accounts from the `/accounts` endpoint, mocks an identity token, and edits your `config.json` properly.
TODO: Update the docs in the repo and revert the PR with the temporary fix.
Test Plan: Tested locally.
Reviewers: bengotow, halla, juan
Reviewed By: halla, juan
Differential Revision: https://phab.nylas.com/D3114
