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.

db/migrate/20210330003356_add_is_deleted_to_comment_votes.rb

@@ -0,0 +1,11 @@
+class AddIsDeletedToCommentVotes < ActiveRecord::Migration[6.1]
+  def change
+    add_column :comment_votes, :is_deleted, :boolean, default: false, null: :false
+    change_column_null :comment_votes, :is_deleted, false
+
+    add_index :comment_votes, :is_deleted, where: "is_deleted = TRUE"
+
+    remove_index :comment_votes, [:user_id, :comment_id], unique: true
+    add_index :comment_votes, [:user_id, :comment_id], unique: true, where: "is_deleted = FALSE"
+  end
+end