Allow account sign in

[WillieCubed]

To secure posts, we should attach them to user accounts. For our "real" MVP, we should support sign in through multiple identity providers:

• Email/password
• Sign in with Google
• Sign in with Facebook
• Sign in with Twitter

For the front-end, this would involve adding appropriately themed UI including sign in buttons for each of the identity providers (@sahanabela can handle this) along with code that connects to our backend's authentication endpoints.

For the back end, we will use JSON Web Tokens (JWT) generated by Firebase Auth to return to the front end.

In the future, we could support sign in with Instagram, but that's more of a stretch goal.