Authorization flow frontend

[connorlbark]

Description

Once the backend authentication flow is established, we need to present a new user with the login screen,
and store the credentials locally on the device.

If we don't have enough information to fully create a valid user object, that is ok - that means for this ticket,
we don't need to actually hit the signup endpoint, but rather just have the frontend for logging in via oauth
and getting the webtoken so that, in a future ticket, we can add a 'signup' form which actually hits the
endpoint.

Perhaps this would work for storing token? https://docs.expo.dev/versions/latest/sdk/securestore/

Spec

• App opens to login authentication iff the user has not authenticated before
• token stored locally in a secure (encrypted) fashion