Previously, the flattening was starting at a very low threshold (above
depth 4) on all screen sizes. This changes it to be responsive, where
larger screens (generally) won't flatten until higher depth levels. On
desktop-size resolutions, it will probably be extremely rare to ever see
any flattened threads, since it will only happen at depth 20+.
These thresholds were picked pretty arbitrarily, but seemed decent from
some testing. I also kept the indentation size smaller until a larger
screen size, since the sidebar's appearance actually reduces the comment
tree space by a lot, and the indents are excessive until the screen is
quite large.