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/entry.rb

@@ -56,6 +56,10 @@ class Entry < ApplicationRecord
     Balance::TrendCalculator.new(self, entries, balances).trend
   end
 
+  def linked?
+    plaid_id.present?
+  end
+
   class << self
     def search(params)
       EntrySearch.new(params).build_query(all)