Implement authorization via Discord

[vm2052]

We're introducing the opportunity to login via Discord (other platforms come later)

1. Create a button to login via Discord on the Sign-in page
2. Call GET /auth/discord endpoint by clicking the button
3. Handle the callback from discord /auth/discord/callback and capture code from the URL (like /auth/discord/callback?code=abc123)
4. Call the existing POST /users/login endpoint by changing the headers to either X-Firebase-Token ( if calling from the normal firebase sign-in) or X-Discord-Code (if doing it via the discord log-in flow)

The API can be found at https://catbytes-community.github.io/webplatform-backend/