Issue #449: Display and change order locale (#459)

* Add more security headers (#458)

* Include some missing security headers

This change adds the following security headers:
* X-Content-Type-Options to prevent content type sniffing
* Referrer-Policy to prevent leaking referrer information when navigating away from the instance

* Migrate from Docker sample to manual configuration

Migrate the additional security headers from the Docker configuration sample to the manual configuration guide.

Add DS_Store to gitingore

* Show order locale in order details

* Add OrderLocaleChange view and OrderLocaleForm

Refactor OrderLocaleForm. Add test

src/pretix/control/urls.py

@@ -111,6 +111,8 @@ urlpatterns = [
             name='event.order.extend'),
         url(r'^orders/(?P<code>[0-9A-Z]+)/contact$', orders.OrderContactChange.as_view(),
             name='event.order.contact'),
+        url(r'^orders/(?P<code>[0-9A-Z]+)/locale', orders.OrderLocaleChange.as_view(),
+            name='event.order.locale'),
         url(r'^orders/(?P<code>[0-9A-Z]+)/comment$', orders.OrderComment.as_view(),
             name='event.order.comment'),
         url(r'^orders/(?P<code>[0-9A-Z]+)/change$', orders.OrderChange.as_view(),