Add custom development header to CORS configuration
In #370 I forgot about the custom development header `x-integreat-development` which is used to differentiate actual traffic from dev traffic in the statistics.

Thus, we have to add 
```
from corsheaders.defaults import default_headers

CORS_ALLOW_HEADERS = list(default_headers) + [
    'x-integreat-development',
]
```
to `src/backend/settings.py`.
diff --git a/src/backend/settings.py b/src/backend/settings.py
index a1f4353a4..852efa48f 100644
--- a/src/backend/settings.py
+++ b/src/backend/settings.py
@@ -9,7 +9,6 @@ https://docs.djangoproject.com/en/2.2/topics/settings/
 For the full list of settings and their values, see
 https://docs.djangoproject.com/en/2.2/ref/settings/
 """
-
 import os
 
 # Build paths inside the project like this: os.path.join(BASE_DIR, ...)
@@ -245,3 +244,17 @@ GVZ_API_ENABLED = True
 
 # Allow access to all domains by setting the following variable to TRUE
 CORS_ORIGIN_ALLOW_ALL = True
+
+# Extend default headers with development header to differenciate dev traffic in statistics
+CORS_ALLOW_HEADERS = [
+    'accept',
+    'accept-encoding',
+    'authorization',
+    'content-type',
+    'dnt',
+    'origin',
+    'user-agent',
+    'x-csrftoken',
+    'x-requested-with',
+    'x-integreat-development',
+]
['src/backend/settings.py']