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).

test/factories/user_upgrade.rb

@@ -4,7 +4,7 @@ FactoryBot.define do
     purchaser { recipient }
     upgrade_type { "gold" }
     status { "pending" }
-    stripe_id { nil }
+    transaction_id { nil }
 
     factory(:self_gold_upgrade) do
       upgrade_type { "gold" }