Integration tests for Rally Web Platform

[rhelmer]

This PR sets up the Firebase emulators, and allows writing jest+typescript tests using Selenium.

It also updates the README with info on how testing works, and how you can use the Firebase emulators for local development (which I definitely recommend, it's quite easy!)

Currently it supports Firefox and signs in (using the emulated Google auth handler) and tests the UX for the site both with and without an extension installed.

I'd like to add a few things before landing:

• add Chrome support
• add screenshot on CI failure
• dump browser/webextension and server logs so we can inspect for correct behavior

However I think it's good enough to review now.

[hamilton]

This looks good to me! I appreciate that we're only using replace in the initialize-firestore module, since we're least likely to be utilizing that when pulling in Storybook.

Had a few small suggestions. Feel free to merge this once those are taken care of.

[Carla-Moz]

Got the emulator working. It's so cool! Also just had one comment about adding the path to the README re the file we have to update locally.