Enable authentication.

[polina-c]

Add authentication with Firebase: https://firebase.google.com/docs/auth/flutter/start

It should be done in minimal simplest way.

Rough UX:

1. User sees circle avatar on very top right
2. User clicks it and sees menu 'Sign In' in drawer (similar to hamburger that you see now instead of menu, when screen is small)
3. User clicks 'Sign In' and goes through sign in process (how Firebase suggests it)
4. When done the icon start showing circle with user's first letter
5. When user clicks the icon they see commands 'Sign Out' and 'Account'

The page 'Account' should contain 'Delete account' and options to be added to contributors and get notifications like it is to what is in form https://forms.gle/GVBRUABAPEHpEQj97.

The feature should be under the flag while in development.

If you want to take this issue, let me know and I will make you contributor here and in Firebase project.