This PR fixes impersonation in local development by fetching the user
from the GitHub API so we can get their `github_user_id`.
The `github_user_id` is now required after #16704.
Since this is just a development flow, we're fetching the user on the
client as opposed to making changes on the server.
This request uses the `GITHUB_TOKEN` environment variable for
authentication, if it exists, or will make an unauthenticated GitHub API
request.
Release Notes:
- N/A
935cf542ae