Previously, Headscale would only use the `email` OIDC
claim to set the Headscale user. In certain cases
(self-hosted SSO), it may be useful to instead use the
`preferred_username` to set the Headscale username.
This also closes juanfont#938.

This adds a config setting to use this claim instead.
The OIDC docs have been updated to include this entry as well.
In addition, this adds an Authelia OIDC example to the docs.

Updated the MockOIDC wrapper to take an environment variable that
lets you set the username/email claims to return.

Added two integration tests, TestOIDCEmailGrant and
TestOIDCUsernameGrant, which check the username by checking the FQDN of
clients.

Updated the HTML template shown after OIDC login to show whatever
username is used, based on the Headscale settings.