Epic Stack OAuth 2.0 example

This demonstrates how to add an OAuth 2.0 provider to your Epic Stack application. It uses GitHub, but the changes would be very similar for any other OAuth 2.0 provider with a remix-auth strategy.

The easiest way to review the changes is to look at the commits. Here are the highlights:

1. Install remix-auth-github and setup a GitHub app as documented in remix-auth-github
2. Add environment variables for the GitHub app's client ID and client secret
3. Add the GitHubStrategy to the available strategies for your authenticator.
4. Add a button to login with GitHub to the login page. It should post to /auth/github.
5. If no user exists for the GitHub email, set the onboardingEmail and redirect the user to onboarding. Otherwise, do the same thing you do when a user signs in with a password.
6. Add an action on a route to handle the /auth/github that calls authenticate with the GitHubStrategy.
7. Add a loader on a route for the /auth/github/callback GitHub callback. It should call authenticate with the GitHubStrategy again. Do the same thing to handle 2FA that we do for verifying password login.