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/forward_calculator.rb

@@ -0,0 +1,28 @@
+class Balance::ForwardCalculator < Balance::BaseCalculator
+  private
+    def calculate_balances
+      current_cash_balance = 0
+      next_cash_balance = nil
+
+      @balances = []
+
+      account.start_date.upto(Date.current).each 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)
+
+        next_cash_balance = if valuation
+          valuation.amount - holdings_value
+        else
+          calculate_next_balance(current_cash_balance, entries, direction: :forward)
+        end
+
+        @balances << build_balance(date, next_cash_balance, holdings_value)
+
+        current_cash_balance = next_cash_balance
+      end
+
+      @balances
+    end
+end