🐛(backend) cast DOCUMENT_IMAGE_MAX_SIZE in integer

The expected type for the settings DOCUMENT_IMAGE_MAX_SIZE is an
integer. By not using django configurations IntegerValue, the value is
used as it and most of the time will be a string. We must use the
IntegerValue in order to cast the value in string.

Author: lunika

src/backend/impress/settings.py

@@ -142,7 +142,7 @@ class Base(Configuration):
     )
 
     # Document images
-    DOCUMENT_IMAGE_MAX_SIZE = values.Value(
+    DOCUMENT_IMAGE_MAX_SIZE = values.IntegerValue(
         10 * (2**20),  # 10MB
         environ_name="DOCUMENT_IMAGE_MAX_SIZE",
         environ_prefix=None,