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.
Factor out FontAwesome icons into a set of helpers. This is so that it's
easier to keep track of which icons we're using and easier to change
icons globally.
