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

test/fixtures/transactions.yml

@@ -0,0 +1,6 @@
+one:
+  category: food_and_drink
+  merchant: amazon
+
+transfer_out: { }
+transfer_in: { }