Add a Shortlink menu option to the comment menu. Clicking this will copy
a DText comment shortlink (e.g. `comment #12345`) to the clipboard. You
can middle-click or right-click the menu option to get the full URL.
The menu option is called Shortlink instead of `comment #1234` because
show the full comment ID in the menu makes the menu look too unbalanced.
Note that the `navigator.clipboard` API can only be used in a https://
environment. It won't work in non-HTTPS development environments. ngrok
can help with this.
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