Remove 'expect-firestore' and use firebase emulator to test security rules

[jamescrowley]

Currently we use expect-firestore, which is a pretty dead library - https://github.com/GitbookIO/expect-firestore.

Instead, we should probably follow the suggestions here: https://firebase.google.com/docs/firestore/security/test-rules-emulator and use the rules emulator.

They will need to run in a NodeJS context rather than client-side context.

There is an example project: https://github.com/firebase/quickstart-nodejs/tree/master/firestore-emulator/javascript-quickstart

Hopefully once this is done, it will give us more views on whether we can use the emulators for further integration-y tests