comments: allow votes to be soft deleted.

Make it so that when a user removes their own vote, the vote is soft
deleted (the is_deleted flag is set) instead of hard deleted.

Changes:

* Add is_deleted flag to comment votes.
* Relax uniqueness constraint so you can have multiple deleted votes on
  the same comment. You can still only have one active vote on the comment.
* Add `soft_delete` method to Deletable concern.

test/components/comment_component_test.rb

@@ -92,5 +92,16 @@ class CommentComponentTest < ViewComponent::TestCase
         end
       end
     end
+
+    context "for a comment with a deleted vote" do
+      should "not treat the vote as active" do
+        @user = create(:user)
+        @vote = create(:comment_vote, user: @user, comment: @comment, is_deleted: true, score: 1)
+        render_comment(@comment, current_user: @user)
+
+        assert_css("article.comment[data-is-upvoted=false]")
+        assert_css("article.comment .comment-upvote-link.inactive-link")
+      end
+    end
   end
 end