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/logical/concerns/deletable.rb

@@ -6,6 +6,14 @@ module Deletable
       scope :active, -> { where(is_deleted: false) }
       scope :deleted, -> { where(is_deleted: true) }
       scope :undeleted, -> { where(is_deleted: false) }
+
+      define_method(:soft_delete) do |**options|
+        update(is_deleted: true, **options)
+      end
+
+      define_method(:soft_delete!) do |**options|
+        update!(is_deleted: true, **options)
+      end
     end
   end
 end