CTFd discord auth

Adds a /discordauth discord oauth endpoint to CTFd, which sends a message to a specified webhook on successful authentication

useful for verifying discord user <-> ctfd user

setup:

1. clone to ctfd plugins folder
2. register discord application https://discord.com/developers/applications
3. get oauth client id, client secret
4. add redirect uri: $ctf_url/discordauth e.g. https://2022.uiuc.tf/discordauth. Go to your Discord Developer Protol, click on the new application, click "Oath2" on the left hand bar, and add a "Redirect" with the uri above.
5. create webhook in discord
6. set appropriate env vars for ctfd: DISCORD_AUTH_WEBHOOK, DISCORD_AUTH_CLIENT_ID, DISCORD_AUTH_SECRET

• preferred: in docker-compose.yml