Adds a flag --disable-password-auth that prevents the password login
endpoint from working unless the user has the "owner" (aka. site admin)
role.
Adds a subcommand `coder server create-admin-user` which creates a user
directly in the database with the "owner" role, the "admin" role in
every organization, and password auth. This is to avoid lock-out
situations where all accounts have the login type set to an identity
provider and nobody can login.
