message_edit: Refactor message edit keydown handler.
message_edit: Refactor message edit keydown handler.

This commit modifies handle_message_row_edit_keydown to use
keydown_util.is_enter_event(). This is a precursor to fixing #22062.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip