update_user_backend: Allow authorized org owners to change user emails.

This adds a new special UserProfile flag can_change_user_emails(disabled
by default) and the ability for changing the email address of users in
the realm via update_user_backend. This is useful for allowing
organizations to update user emails without needing to set up a SCIM
integration, but since it gives the ability to hijack user accounts, it
needs to be behind this additional permission and can't be just given to
organization owners by default. Analogical to how the
create_user_backend endpoint works.

zerver/openapi/zulip.yaml

@@ -12779,6 +12779,16 @@ paths:
                     type: object
                   example:
                     [{"id": 4, "value": "0"}, {"id": 5, "value": "1909-04-05"}]
+                new_email:
+                  description: |
+                    New email address for the user. Requires the user making
+                    the request to be an organization administrator and
+                    additionally have the `.can_change_user_emails` special
+                    permission.
+
+                    **Changes**: New in Zulip 10.0 (feature level 313).
+                  type: string
+                  example: username@example.com
             encoding:
               role:
                 contentType: application/json