This is in response to a support ticket where the user had a closed left
sidebar, had added an organization, and then couldn't figure out how to
switch organizations. They had googled and found "The desktop app makes it
easy to switch between different organizations" in our help docs, which was
not sufficiently helpful.
This changes the border-radius to 6px for the tabbed display, which is not
in line with the current Zulip style for border-radius (4px). However 6px
really looks a lot better for this (possibly because it's a bigger box than
most of our other boxes?)
This is a major upgrade, and requires some significant compatibility
work:
* Migrating the pattern-removal logic to use the Registry feature.
* Handling the removal of positional arguments in markdown extensions.
* Handling the removal of safe mode.
