create auth flow to capture Discord, GitHub usernames for contributor automation

[josefaidt]

Describe the Feature

Create an auth flow that would push folks through authenticating with Discord, then linking their GitHub account. This will enable automations where we are posting GitHub information such as a PR merge in Discord and would like to reference their Discord username

The ideal flow would look like:

1. click this link, user is taken to bot.amplify.aws auth flow page
2. log in with Discord
3. log in with GitHub
4. display "thanks for linking your accounts! it is now safe to close this page"

This flow's landing page or public messaging should be very clear in our intentions for capturing this information