Figure out how to emulate firebase in integration tests

[itegulov]

In the effort to untie ourselves from depending on validTokens (see #76) we should figure out a way to set up a local firebase project. https://firebase.google.com/docs/emulator-suite might be useful.

[volovyks]

I set up and tried using the Firebase emulator, but It appeared that it was not signing JWT Tokens :/
https://firebase.google.com/docs/emulator-suite/connect_auth#id_tokens
I simulate almost everything from real Firebase but not what we need.