User creation and authentication control

[rfay]

The current setup is unsustainable, of course: Anybody with a github account can create an account and then unlimited workspaces. It was set up and put in place for Drupalcon Chicago this coming week, and so far has not resulted in disaster.

We need to control account creation.

Probably:

• Precreate users
• Use github authentication

CODER_OAUTH2_GITHUB_ALLOW_SIGNUPS=true
CODER_OAUTH2_GITHUB_ALLOWED_ORGS=ddev,my-other-org
# leave password auth enabled for manually created exception users
# do NOT set CODER_DISABLE_PASSWORD_AUTH=true