✨(frontend) allow parametrization of the transcrip document destination

Not all self-hosted instances will configure this setting, so a default text is
shown when the destination is unknown.

This is important to let users quickly click the link and understand which
platform is used to handle the transcription documents.

src/backend/meet/settings.py

@@ -345,6 +345,9 @@ class Base(Configuration):
         "manifest_link": values.Value(
             None, environ_name="FRONTEND_MANIFEST_LINK", environ_prefix=None
         ),
+        "transcription_destination": values.Value(
+            None, environ_name="FRONTEND_TRANSCRIPTION_DESTINATION", environ_prefix=None
+        ),
     }
 
     # Mail