Improve use of extra terminal width (avoid wide message view)

[neiljp]

I'm filing this to track @aero31aero 's work on #388 in the past, and potential areas to improve in a similar way.

I agree with the main thrust of the original PR, ie. to avoid having a central column which reaches very large widths, which can make it difficult to read messages.

The original PR does deviate from the webapp style, in that it puts space between the side columns and the central one - that feels strange to me, so I'd suggest we discuss this before proceeding further in that direction. Perhaps we could put some between the columns but most to either side?

Another area to consider is how we could actively use the extra space. My only concern with taking this route is that this could lead us to focus on improvements for only users who use ZT with large terminal widths. That said:

• The side panel widths are fixed width right now; we could scale them if there is spare space, at least up to some limit
• We could have extra columns/panels for showing contextual information, not just in popups, eg. topics, attachments, ...
• With a lot of refactoring we might be able to support showing multiple active narrows (which could be compressed into a tabbed view on narrower terminals)

This first came up in #zulip-terminal>transparency with some discussion followed up on #388 in #zulip-terminal>#T388 Wide Mode, though there may have been other topics covering this.