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.

app/controllers/comment_votes_controller.rb

@@ -13,7 +13,11 @@ class CommentVotesController < ApplicationController
 
     @comment.with_lock do
       @comment_vote = authorize CommentVote.new(comment: @comment, score: params[:score], user: CurrentUser.user)
-      CommentVote.where(comment: @comment, user: CurrentUser.user).destroy_all
+
+      CommentVote.active.where(comment: @comment, user: CurrentUser.user).each do |vote|
+        vote.soft_delete!(updater: CurrentUser.user)
+      end
+
       @comment_vote.save
     end
 
@@ -22,8 +26,9 @@ class CommentVotesController < ApplicationController
   end
 
   def destroy
-    @comment_vote = authorize CommentVote.find_by!(comment_id: params[:comment_id], user: CurrentUser.user)
-    @comment_vote.destroy
+    # XXX should find by comment vote id.
+    @comment_vote = authorize CommentVote.active.find_by!(comment_id: params[:comment_id], user: CurrentUser.user)
+    @comment_vote.soft_delete(updater: CurrentUser.user)
 
     respond_with(@comment_vote)
   end