Feat account linking

[Arka-cell]

It seems that linking providers would make it really hard to work with drf-firebase-auth. This branche is supposed to find a solution for this.

[Arka-cell]

@garyburgmann The app now works fine, instead of relying on email it is trying to get user by UID. This is better, it will handle user by the an unique identifier, and if they update their email or Phone on Firebase, it won't need to be updated in the User model. And, provider linking between Facebook or Google with Email is going to be straightforward.