web: create preview pane

[hyu]

I tried tab-like buttons for "Logs" and "Preview" view options.

• In this PR, it's not obvious the view options are buttons. I'll address this in my next design pass.
• When we have more than two views, this design will have to change.

Nav Design Alternatives considered

I mocked up a design with the view options to the left of the sidebar, but it was a bit too visually obtrusive.

[jazzdan]

LGTM!

[nicks]

one major bug:
when the logs pane is focused, you can scroll with the mousewheel but there's no scrollbar.
when the logs pane is not focused, you can't scroll at all 😭 . and still no scrollbar

some minor issues:
when looking at all logs, and clicking the "Preview" button, i get a "No match" error
when looking at (Tiltfile) logs, and clicking the "Preview" button, I get a blank screen
when the right sidebar is taller than the screen, there's no scrollbar. there used to be a scrollbar before this PR, so this appears to be a regression