Cleanup and tweaks

[andyhite]

This does a handful of things:

• Removes the rtdb version of the extension
• Updates the firestore version with the logging approach used in the rtdb version
• Removes lerna – I was having issues with lerna not playing nicely with the most recent versions of yarn, and there's really no benefit to using lerna
• Adds some basic development setup documentation
• Fixes some things in the extension.yaml for address validation (there were still references to the email extension)
• Uses the extension-specific emulator tools instead of the generic firebase ones

I'm open to discussing any of the changes here, but this was what I needed to do to clean things up, consolidate some stuff, and get the extension working in my local emulator.

[andyhite]

Going to merge this without review since Patrick isn't around and I want to get this on master ASAP