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
Either shows an informational dialog telling you the name has changed,
or a more naggy one if the user needs to log in on a different URL.
The new URL (if any) is hardcoded based on the current URL, and also
with a bonus config param in case other deployments need to do similar.
