comments: add dtext shortlink option to comment menu.

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.

app/helpers/icon_helper.rb

@@ -135,4 +135,8 @@ module IconHelper
   def flag_icon(**options)
     icon_tag("fas fa-flag", **options)
   end
+
+  def link_icon(**options)
+    icon_tag("fas fa-link", **options)
+  end
 end