Support moving a message between topics and streams

[gnprice]

I.e., this Zulip feature:
https://zulip.com/help/rename-a-topic
https://zulip.com/help/move-content-to-another-topic
https://zulip.com/help/move-content-to-another-stream

This might end up being several different features in the UI, just like it is in the web app (as reflected by having several separate pages about it in the Help Center). So we might split this issue up more finely after we've thought through what we want in that respect.

This is a feature we don't have in the zulip-mobile RN app.

Related features:

• compose: Support editing an already-sent message #126
• Show edited/moved marker on messages #171