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/account_import.rb

@@ -20,7 +20,7 @@ class AccountImport < Import
           currency: row.currency,
           date: Date.current,
           name: "Imported account value",
-          entryable: Account::Valuation.new
+          entryable: Valuation.new
         )
       end
     end