Stores tokens in Redis

[simonbs]

This PR rewrites our authentication logic to store access tokens and refresh tokens in Redis.

The motivation is to make way fewer calls to Auth0's Management API which has strict rate limits. Previously we would make a call to the Management API every time we needed the GitHub access token as it was not stored locally. Now we only call the Management API when the user logs in.

As a great side effect the portal has now become slightly faster as it makes fewer network calls.

The flow when logging in is now as follows:

1. Catch the call to /api/auth/callback to exchange Auth0's access token and refresh token for authentication with GitHub for tokens we own.
2. Store the newly created GitHub tokens in Redis.
3. Use the tokens in Redis going forward.

When using the tokens we rely on the expiry dates to determine if we should renew the tokens or now. At this point we do not invoke the Github API with credentials and examine the response to determine if we should refresh the credentials. This was left out as I'm unsure it's needed when we can rely on expiry dates. If the user have revoked the tokens there is nothing we can do anyway to renew them.