Full end-to-end integration tests for Twitter (OAuth 1) and GitHub (OAuth 2) using Puppeteer and Mocha.

This replaces Cypress tests due to issues with Cypress not being able to run tests against external URLs, which we need for our integration tests.

The integration test runner is hosted outside of GitHub Actions (it cannot be hosted by GitHub or on AWS due to IP access controls placed on sign in by providers like Twitter and GitHub) and so the integration tests may not pass if the test runner is offline. If this happens, tests can be re-run later when the test runner is available.

See Pull Request #641 for details.