Plaid sync domain improvements (#2267)

Breaks our Plaid sync process out into more manageable classes. Notably, this moves the sync process to a distinct, 2-step flow:

1. Import stage - we first make API calls and import Plaid data to "mirror" tables
2. Processing stage - read the raw data, apply business rules, build internal domain models and sync balances

This provides several benefits:

- Plaid syncs can now be "replayed" without fetching API data again
- Mirror tables provide better audit and debugging capabilities
- Eliminates the "all or nothing" sync behavior that is currently in place, which is brittle

app/models/credit_card.rb

@@ -1,6 +1,10 @@
 class CreditCard < ApplicationRecord
   include Accountable
 
+  SUBTYPES = {
+    "credit_card" => { short: "Credit Card", long: "Credit Card" }
+  }.freeze
+
   class << self
     def color
       "#F13636"