Implement Sign In and Sign Out Workflows

[bobertoyin]

We want users to be able to sign in/out in order to properly attribute reviews, along with allowing personalization in the future.

This development is two-fold: we need to integrate with the WCA to authorize users, then use server-side sessions to keep users logged in, allow them to log out, and allow our back-end to determine the current user.

You'll want to read up on WCA's v0 API documentation to understand what routes you'll need to add, along with Flask Session's documentation to implement server-side sessions. Lastly, we'll use Flask Sessions' SQLAlchemy interface to avoid bringing in any more dependencies.