✨(back) allow theme customnization using a configuration file

We want to customize the theme by using a configuration file. This
configuration file path can be defined using the settings
THEME_CUSTOMIZATION_FILE_PATH. If this file does not exists or is an
invalid json, an empty json object will be added in the config endpoint.

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to
 
 ## Added
 
+- ✨(back) allow theme customnization using a configuration file #948
 - ✨ Add a custom callout block to the editor #892
 - 🚩(frontend) version MIT only #911
 - ✨(backend) integrate maleware_detection from django-lasuite #936