user_settings: Add field web_suggest_update_timezone.

This field tracks whether the user should be shown an
alert offering to update their profile time zone to the
time zone of the browser in case they differ.

The field is added to RealmUserDefault and UserProfile
models with a default value of True.

Fixes part of #16957

zerver/views/user_settings.py

@@ -328,6 +328,7 @@ def json_change_settings(
     ]
     | None = None,
     web_navigate_to_sent_message: Json[bool] | None = None,
+    web_suggest_update_timezone: Json[bool] | None = None,
 ) -> HttpResponse:
     # UserProfile object is being refetched here to make sure that we
     # do not use stale object from cache which can happen when a