BasicAuth: Allow disabling sending WWW-Authenticate header

[agrafix]

Right now, the BasicAuth module sends the WWW-Authenticate header by default via servant-server. This isn't always desirable, since, when called via AJAX, it causes the browser to surface a login prompt on authentication failure.

This PR makes that behavior configurable.

[akhesaCaro]

Hi agrafix,

Thanks for your contribution. The CI has been fixed, could you rebase your branch to see if it passes the CI checks?

[tchoutri]

@agrafix Hi! Unfortunately we lost CI logs, could you please rebase your branch and push it again? :)