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/components/comment_component/comment_component.js

@@ -1,9 +1,12 @@
+import Utility from "../../javascript/src/javascripts/utility.js";
+
 class CommentComponent {
   static initialize() {
     if ($("#c-posts #a-show, #c-comments").length) {
       $(document).on("click.danbooru.comment", ".edit_comment_link", CommentComponent.showEditForm);
       $(document).on("click.danbooru.comment", ".expand-comment-response", CommentComponent.showNewCommentForm);
       $(document).on("click.danbooru.comment", ".unhide-comment-link", CommentComponent.unhideComment);
+      $(document).on("click.danbooru.comment", ".comment-copy-link", CommentComponent.copyLink);
     }
   }
 
@@ -26,8 +29,20 @@ class CommentComponent {
     $comment.find(".body").show();
     e.preventDefault();
   }
-}
 
+  static async copyLink(e) {
+    let $comment = $(this).closest(".comment");
+    let link = `comment #${$comment.data("id")}`;
+    e.preventDefault();
+
+    try {
+      await navigator.clipboard.writeText(link);
+      Utility.notice(`Copied ${link} to clipboard.`);
+    } catch (error) {
+      Utility.error("Couldn't copy link to clipboard");
+    }
+  }
+}
 
 $(document).ready(CommentComponent.initialize);