Allow overriding AdminSite site_header

[mingfeng]

django-helusers overrides the AdminSite and change the site_header to be a read-only property (see below), which prevents the client code assigning a custom site header to it.

    @property
    def site_header(self):
        if 'django.contrib.sites' in settings.INSTALLED_APPS:
            Site = apps.get_model(app_label='sites', model_name='Site')
            site = Site.objects.get_current()
            site_name = site.name
        elif hasattr(settings, 'WAGTAIL_SITE_NAME'):
            site_name = settings.WAGTAIL_SITE_NAME
        else:
            return ugettext_lazy("Django admin")
        return ugettext_lazy("%(site_name)s admin") % {'site_name': site_name}

While it's possible to set WAGTAIL_SITE_NAME setting variable to change the admin site header, but it's confusing for those projects that does not use WAGTAIL.

It would be nice to add a set property for site_header also.

[akikoskinen]

Isn't it possible to extend helusers.admin_site.AdminSite and write your own site_header property? If that's too cumbersome or otherwise not desirable, I guess, contributions are welcome.