✨(backend) add server-to-server API endpoint to create documents

We want trusted external applications to be able to create documents
via the API on behalf of any user. The user may or may not pre-exist
in our database and should be notified of the document creation by
email.

src/backend/impress/settings.py

@@ -65,6 +65,7 @@ class Base(Configuration):
     # Security
     ALLOWED_HOSTS = values.ListValue([])
     SECRET_KEY = values.Value(None)
+    SERVER_TO_SERVER_API_TOKENS = values.ListValue([])
 
     # Application definition
     ROOT_URLCONF = "impress.urls"
@@ -502,6 +503,26 @@ class Base(Configuration):
         "day": 200,
     }
 
+    # Conversion microservice
+    CONVERSION_API_KEY = values.Value(
+        environ_name="CONVERSION_API_KEY",
+        environ_prefix=None,
+    )
+    CONVERSION_API_URL = values.Value(
+        environ_name="CONVERSION_API_URL",
+        environ_prefix=None,
+    )
+    CONVERSION_API_CONTENT_FIELD = values.Value(
+        default="content",
+        environ_name="CONVERSION_API_CONTENT_FIELD",
+        environ_prefix=None,
+    )
+    CONVERSION_API_TIMEOUT = values.Value(
+        default=30,
+        environ_name="CONVERSION_API_TIMEOUT",
+        environ_prefix=None,
+    )
+
     # Logging
     # We want to make it easy to log to console but by default we log production
     # to Sentry and don't want to log to console.