upgrades: rename stripe_id to transaction_id

* Rename the stripe_id column to transaction_id.
* Add a new payment_processor column to identity the processor used for
  this transaction (and hence, which backend system the transaction_id is for).

db/migrate/20220514175125_rename_stripe_id_to_transaction_id_on_user_upgrades.rb

@@ -0,0 +1,7 @@
+class RenameStripeIdToTransactionIdOnUserUpgrades < ActiveRecord::Migration[7.0]
+  def change
+    add_column :user_upgrades, :payment_processor, :integer, null: false, default: 0
+    rename_column :user_upgrades, :stripe_id, :transaction_id
+    add_index :user_upgrades, :payment_processor
+  end
+end