This replaces all chevrons with a single icon, affecting components like the
room list, network dropdown, and rebrand modal.
Rather than having files for each direction that can get out of sync, this
change uses one direction and rotates in CSS for the others.
Fixes https://github.com/vector-im/riot-web/issues/14663
We don't need columns of divs to equally size themselves, so use easier layout techniques to make the list fit in the container. We have to take a hit with `height:100%`, but the hit is much more insignificant than confusing the layout engine.
The layout engine has a hard time with dynamically-but-statically-sized stuff like `width: 100%; display: flex;`, particularly when it is nested so badly. Overall this should improve performance for the app by not having to re-paint so often.
Fixes https://github.com/vector-im/riot-web/issues/14639
