votes: add is_deleted flag to post_votes table.

Add an is_deleted flag to post_votes so they can be soft-deleted in the future.

db/migrate/20211121080239_add_is_deleted_to_post_votes.rb

@@ -0,0 +1,9 @@
+class AddIsDeletedToPostVotes < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+
+  def change
+    add_column :post_votes, :is_deleted, :boolean, default: false, null: :false
+    add_index :post_votes, :is_deleted, where: "is_deleted = TRUE", algorithm: :concurrently
+    add_index :post_votes, [:user_id, :post_id], unique: true, where: "is_deleted = FALSE", algorithm: :concurrently
+  end
+end