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/forms/orders.py

@@ -1,4 +1,5 @@
 from django import forms
+from django.conf import settings
 from django.core.exceptions import ValidationError
 from django.db import models
 from django.utils.formats import localize
@@ -118,3 +119,15 @@ class OrderContactForm(forms.ModelForm):
     class Meta:
         model = Order
         fields = ['email']
+
+
+class OrderLocaleForm(forms.ModelForm):
+    locale = forms.ChoiceField()
+
+    class Meta:
+        model = Order
+        fields = ['locale']
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.fields['locale'].choices = [(a, a) for a in self.instance.event.settings.locales]