Allow hiding custom login UI even if an htpasswd file is provided.

We have a few apps behind the `google_auth_proxy` that are configured to use an `htpasswd` file for API access. Folks on the team have found the login UI confusing because the `htpasswd` we're providing is only for API access, but providing an `htpasswd` file causes the login UI to display a username / password form along with the "Login with Google" button.

This PR adds a flag called `display-htpasswd-form`. It defaults to `true` so that the current behavior remains the same. If the flag is set to false, the username / password fields on the login page will not be displayed even if an htpasswd file is provided.

Let me know if this is something you're interested in pulling in.