Account::Sync model and test fixture simplifications (#968)

* Add sync model

* Fresh fixtures for sync tests

* Sync tests overhaul

* Fix entry tests

* Complete remaining model test updates

* Update system tests

* Update demo data task

* Add system tests back to PR checks

* More simplifications, add empty family to fixtures for easier testing

app/models/family.rb

@@ -104,7 +104,7 @@ class Family < ApplicationRecord
     Money.new(accounts.active.liabilities.map { |account| account.balance_money.exchange_to(currency, fallback_rate: 0) }.sum, currency)
   end
 
-  def sync_accounts
-    accounts.each { |account| account.sync_later if account.can_sync? }
+  def sync(start_date: nil)
+    accounts.active.sync(start_date: start_date)
   end
 end