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.rb

@@ -0,0 +1,9 @@
+class Balance < ApplicationRecord
+  include Monetizable
+
+  belongs_to :account
+  validates :account, :date, :balance, presence: true
+  monetize :balance
+  scope :in_period, ->(period) { period.nil? ? all : where(date: period.date_range) }
+  scope :chronological, -> { order(:date) }
+end