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.
