Fix #294 -- Blacklist for slug validation (#325)

* Fix #294 - blacklist for slug validation

* Fix #294 - blacklist for slug validation

* fix for failing test

src/pretix/base/validators.py

@@ -0,0 +1,48 @@
+from django.core.exceptions import ValidationError
+from django.utils.translation import ugettext_lazy as _
+
+
+class BlacklistValidator:
+
+    blacklist = []
+
+    def __call__(self, value):
+        # Validation logic
+        if value in self.blacklist:
+            raise ValidationError(
+                _('This slug has an invalid value: %(value)s.'),
+                code='invalid',
+                params={'value': value},
+            )
+
+
+class EventSlugBlacklistValidator(BlacklistValidator):
+
+    blacklist = [
+        'download',
+        'healthcheck',
+        'locale',
+        'control',
+        'redirect',
+        'jsi18n',
+        'metrics',
+        '_global',
+        '__debug__'
+    ]
+
+
+class OrganizerSlugBlacklistValidator(BlacklistValidator):
+
+    blacklist = [
+        'download',
+        'healthcheck',
+        'locale',
+        'control',
+        'pretixdroid',
+        'redirect',
+        'jsi18n',
+        'metrics',
+        '_global',
+        '__debug__',
+        'about'
+    ]