message_edit: Send only changed settings in event data and api response.

Previously, we included all three message edit related settings
("allow_message_editing", "message_content_edit_limit_seconds" and
"edit_topic_policy") in the event data and api response irrespective
of which of these settings were changed. Now, we only include changed
settings and separate events are sent for each setting if more than
one of them is changed.

Note that the previous typed in event_schema.py for
`message_content_edit_limit_seconds` incorrectly did not allow `None`
as a value, which is used to encode no limit.

docs/tutorials/new-feature-tutorial.md

@@ -481,7 +481,7 @@ with the new value. E.g., for `authentication_methods`, we created
 
 # import do_set_realm_authentication_methods from actions.py
 from zerver.actions.realm_settings import (
-    do_set_realm_message_editing,
+    do_reactivate_realm,
     do_set_realm_authentication_methods,
     # ...
 )