Sometimes it may be useful to run WebStatus through standalone
webserver that may be configured for HTTP authentication (for
example in corporate network it may use LDAP to authenticate users).
In such cases it's reasonable to trust webserver and instead of
asking for username and password for actions like trigger build,
just get username from HTTP headers.
In case if username is already known from HTTP request headers, there
is no sense to ask user to enter it again. This adds needUserForm
method to Authz class and modifies the authFormIfNeeded template to
hide the username field if not needed.