Previews when composing a message

[gnprice]

This is a useful feature in web:
https://zulip.com/help/preview-your-message-before-sending
and it would be useful in mobile too.

(It's not as common to want it in mobile, I think, because one tends to stick to shorter messages with simpler formatting. But that's far from absolute.)

[gnprice]

We had a user request for this the other day:
https://chat.zulip.org/#narrow/channel/48-mobile/topic/iOS.20feedback.20for.20Julia.20community.20member/near/1961857
https://chat.zulip.org/#narrow/channel/48-mobile/topic/iOS.20feedback.20for.20Julia.20community.20member/near/1963803

They raised code blocks and LaTeX as particularly useful situations for a preview.