This does not rely on the desktop app being able to register for the
zulip:// scheme (which is problematic with, for example, the AppImage
format).
It also is a better interface for managing changes to the system,
since the implementation exists almost entirely in the server/webapp
project.
This provides a smoother user experience, where the user doesn't need
to do the paste step, when combined with
https://github.com/zulip/zulip-desktop/pull/943.
Fixes#13613.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
a552c2e5f9