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/base_calculator.rb

@@ -0,0 +1,35 @@
+class Balance::BaseCalculator
+  attr_reader :account
+
+  def initialize(account)
+    @account = account
+  end
+
+  def calculate
+    Rails.logger.tagged(self.class.name) do
+      calculate_balances
+    end
+  end
+
+  private
+    def sync_cache
+      @sync_cache ||= Balance::SyncCache.new(account)
+    end
+
+    def build_balance(date, cash_balance, holdings_value)
+      Balance.new(
+        account_id: account.id,
+        date: date,
+        balance: holdings_value + cash_balance,
+        cash_balance: cash_balance,
+        currency: account.currency
+      )
+    end
+
+    def calculate_next_balance(prior_balance, transactions, direction: :forward)
+      flows = transactions.sum(&:amount)
+      negated = direction == :forward ? account.asset? : account.liability?
+      flows *= -1 if negated
+      prior_balance + flows
+    end
+end