settings: Move 'enter_sends' setting to property_types dict.

This commit moves "enter_sends" setting to property_types dict.
With this change, changing enter_sends setting also sends an
event of type "update_display_settings" and thus enables us
to live-update the UI.

zerver/lib/events.py

@@ -502,6 +502,10 @@ def fetch_initial_state_data(
 
     if want("update_display_settings"):
         for prop in UserProfile.property_types:
+            if prop == "enter_sends":
+                # This will be removed when we make the API change to
+                # move this to the update_display_settings event type.
+                continue
             state[prop] = getattr(settings_user, prop)
         state["emojiset_choices"] = UserProfile.emojiset_choices()
         state["timezone"] = settings_user.timezone