Account:: namespace simplifications and cleanup (#2110)

* Flatten Holding model

* Flatten balance model

* Entries domain renames

* Fix valuations reference

* Fix trades stream

* Fix brakeman warnings

* Fix tests

* Replace existing entryable type references in DB

app/models/balance/reverse_calculator.rb

@@ -0,0 +1,32 @@
+class Balance::ReverseCalculator < Balance::BaseCalculator
+  private
+    def calculate_balances
+      current_cash_balance = account.cash_balance
+      previous_cash_balance = nil
+
+      @balances = []
+
+      Date.current.downto(account.start_date).map do |date|
+        entries = sync_cache.get_entries(date)
+        holdings = sync_cache.get_holdings(date)
+        holdings_value = holdings.sum(&:amount)
+        valuation = sync_cache.get_valuation(date)
+
+        previous_cash_balance = if valuation
+          valuation.amount - holdings_value
+        else
+          calculate_next_balance(current_cash_balance, entries, direction: :reverse)
+        end
+
+        if valuation.present?
+          @balances << build_balance(date, previous_cash_balance, holdings_value)
+        else
+          @balances << build_balance(date, current_cash_balance, holdings_value)
+        end
+
+        current_cash_balance = previous_cash_balance
+      end
+
+      @balances
+    end
+end