🔧(env) disable the DINUM-specific footer by default

the footer used is very specific to the DINUM/French gov instance so it
should not be enabled by default for everyone.

it's still a bit weird to keep this footer in the code here but at least
it removes the issue easily. any PR to clean the code is appreciated :)

src/backend/meet/settings.py

@@ -324,6 +324,9 @@ class Base(Configuration):
         "feedback": values.DictValue(
             {}, environ_name="FRONTEND_FEEDBACK", environ_prefix=None
         ),
+        "use_french_gov_footer": values.BooleanValue(
+            False, environ_name="FRONTEND_USE_FRENCH_GOV_FOOTER", environ_prefix=None
+        ),
     }
 
     # Mail