Automatic update checks (#434)

* Basic update checks

* Fix issues pointed out by @rixx

* First test

* Add tests

* Even more tests

src/pretix/control/forms/global_settings.py

@@ -1,5 +1,6 @@
 from collections import OrderedDict
 
+from django import forms
 from django.utils.translation import ugettext_lazy as _
 from i18nfield.forms import I18nFormField, I18nTextInput
 
@@ -32,3 +33,26 @@ class GlobalSettingsForm(SettingsForm):
             for key, value in response.items():
                 # We need to be this explicit, since OrderedDict.update does not retain ordering
                 self.fields[key] = value
+
+
+class UpdateSettingsForm(SettingsForm):
+    update_check_perform = forms.BooleanField(
+        required=False,
+        label=_("Perform update checks"),
+        help_text=_("During the update check, pretix will report an anonymous, unique installation ID, "
+                    "the current version of pretix and your installed plugins and the number of active and "
+                    "inactive events in your installation to servers operated by the pretix developers. We "
+                    "will only store anonymous data, never any IP adresses and we will not know who you are "
+                    "or where to find your instance. You can disable this behaviour here at any time.")
+    )
+    update_check_email = forms.EmailField(
+        required=False,
+        label=_("E-mail notifications"),
+        help_text=_("We will notify you at this address if we detect that a new update is available. This "
+                    "address will not be transmitted to pretix.eu, the emails will be sent by this server "
+                    "locally.")
+    )
+
+    def __init__(self, *args, **kwargs):
+        self.obj = GlobalSettingsObject()
+        super().__init__(*args, obj=self.obj, **kwargs)