comments: rework deleted comments.

Let users see when a post has deleted comments. Show normal users a
'[deleted]' placeholder when a comment is deleted. Show the full comment
to moderators.

Also fix it so that the comment creator can't edit or undelete deleted
comments, and users can't vote on or report deleted comments.

Finally, hide the creator_id, updater_id, and body of deleted comments
in the API.

test/components/comment_component_test.rb

@@ -26,6 +26,29 @@ class CommentComponentTest < ViewComponent::TestCase
       end
     end
 
+    context "for a deleted comment" do
+      setup do
+        @deleted_comment = as(create(:user)) { create(:comment, is_deleted: true) }
+      end
+
+      should "have the creator and body hidden for a Member" do
+        render_comment(@deleted_comment, current_user: @deleted_comment.creator)
+
+        assert_css("article[data-is-dimmed=true]")
+        assert_css("article .author-name", text: "[deleted]")
+        assert_css("article .body p", text: "[deleted]")
+      end
+
+      should "be visible for a Moderator" do
+        render_comment(@deleted_comment, current_user: create(:moderator_user))
+
+        assert_css("article[data-is-dimmed=true]")
+        assert_no_css("article .unhide-comment-link")
+        assert_css("article .author-name", text: @deleted_comment.creator.pretty_name)
+        assert_css("article .body p", text: @deleted_comment.body)
+      end
+    end
+
     context "for a comment with moderation reports" do
       should "show the report notice to moderators" do
         create(:moderation_report, model: @comment)