This new setting controls whether or not users are allowed to see the
edit history in a Zulip organization. It controls access through 2
key mechanisms:
* For long-ago edited messages, get_messages removes the edit history
content from messages it sends to clients.
* For newly edited messages, clients are responsible for checking the
setting and not saving the edit history data. Since the webapp was
the only client displaying it before this change, this just required
some changes in message_events.js.
Significantly modified by tabbott to fix some logic bugs and add a
test.
Durga Akhil Mundroy