This will allow clients to display MOVED/EDITED indicators, and their
tooltips, without interacting with the `edit_history` section of
message objects, which we plan to remove in the future.
Supporting that requires both introducing both last_moved_timestamp,
and changing the definition of last_edit_timestamp to not include
message moves, which involves recalculating it at the API layer.
The last_moved_timestamp is not present if the topic moves for the
message are for resolving or unresolving the topic. It is always
present for channel moves.
Co-authored-by: Lauryn Menard <lauryn@zulip.com>