ui: Restrict max width of center column for easier reading.

[aero31aero]

Adds a config option layout=space-between to make the center column fix width and add space between the three columns on wider screens. On smaller screens, the original behavior is preserved.

This mimics the webapp's idea of having the line-length be small so that the text is easy to read even on larger screens.

Screenshots:

1. Small Width:

2. Medium Width:

3. Large Width:

[aero31aero]

@neiljp should I rebase this or this feature isn't needed now?

[neiljp]

@aero31aero I would suggest seeing what people think; I'm not sure how much work is necessary to rebase? We could discuss on #zulip-terminal (did we already?), and/or open an issue? I expect something like this could definitely be useful, though I don't normally have a window that large personally, but I'm not sure where people would prefer the 'scaling' to take place: between the panels and the message list, or on the outside of the panels? Or maybe even filling extra space with the 'topics' view all the time, rather than toggling the view...?

[zulipbot]

Heads up @aero31aero, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.