Add Login button to the app-bar

[luciddr34m3r]

Currently, a logout button appears when you are logged in, but no login button appears when you aren't logged in.

The AuthButton component doesn't render a button when logged out. It probably makes sense to restructure it so that it can log you in as well.

• [ ] If a user isn't logged in, render a button that says 'log in'
• [ ] When clicked, that button creates a popup dialogue that presents you with the auth form