Support moving messages between streams

[rk-for-zulip]

zulip/zulip#13912 introduces the ability to move a message from one stream to another via a new optional new_stream_id: number key on update_message events. The mobile app will need to handle these.

See also relevant discussion on CZO.

[rk-for-zulip]

Known issues in implementing this include the fact that the set of known Messages for any stream is pervasively assumed to be a contiguous range. Moving messages from one stream to another may break this invariant.

[rk-for-zulip]

Additional information from @timabbott (slightly edited to include cross-project GitHub links):

The server commits zulip/zulip@843345d and zulip/zulip@7990676 (as part of zulip/zulip#13912 / zulip/zulip#6427) added support in the Zulip server and webapp for a new variant of the update_message event type, with a new_stream_id field. It should work the same as the existing support for changing a topic (including the message_ids field with additional messages to move), except that the stream changed as well.

I imagine these will be relatively rare, but it also didn't end up being a huge amount of work to support this in the webapp's event handling.

Like with the webapp, the part of updating unread count data structures is likely the most important thing here.