Sale History, New Tests, Rates

Sale History -

New app. Contains:
BusinessSaleHistory:
-  document --> FKey to Document which must be an order
ItemSaleHistory:
-  line --> FKey to a DocumentLine which must be of an order

Added signals in Document and DocumentLine addition/ deletion to add
BusinessSaleHistory or ItemSaleHistory accordingly. Has a condition to
validate it belongs to an order only.

New Tests:

Removed CRUD tests... DRF handles that so I do not need to check what's
already checked.

Businesses - Tests for creating client and supplier (see if is_client or
is_supplier are indeed True)
also test to check sale history is working correctly.

Items - Test for checking sale history is working correctly.

Rates:
Created Rates app for VAT and Dollar rates.
Nothing important or interesting here...