feat(grfn/bbbg): Allow Organizers to sign in via Discord

Allow users with the Organizers role to sign in via a Discord Oauth2
handshake, creating a user in the users table and adding the ID of that
user to the session.

Change-Id: I39d9e17433e71b07314b9eabb787fb9214289772
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4409
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Autosubmit: grfn <grfn@gws.fyi>

users/grfn/bbbg/resources/migrations/20211212165646-init-schema.up.sql

@@ -26,6 +26,7 @@ CREATE TABLE "event_attendee" (
 -- ;;
 CREATE TABLE "user" (
     "id" UUID PRIMARY KEY NOT NULL DEFAULT uuid_generate_v4(),
+    "username" TEXT NOT NULL,
     "discord_user_id" TEXT NOT NULL,
     "created_at" TIMESTAMP WITHOUT TIME ZONE NOT NULL DEFAULT now()
 );