Collapse long messages (with "show more")

[gnprice]

Zulip web automatically collapses (or "condenses") messages that are very tall compared to the viewport, and offers a "show more" button. Some background discussion here:
https://chat.zulip.org/#narrow/stream/101-design/topic/when.20to.20condense.20messages/near/1934704

We should perhaps do something like this in mobile.

It's uncommon in my experience that I feel the need for this feature in mobile — just today I ran into it for perhaps the first time in a year or more. That's probably a sign that whatever we do should be less vigorous than the rule we have in the web app, if we do this at all.

This is a post-launch issue because it's something the legacy app doesn't do.