Reparación de errores de redondeo (#4)

* adds comments to create_json_openfactura()

* changes round in item price calculation to include 4 decimals

* changes in net value calculation to avoid loss of precision

* changes IndMntNeto to include IVA on items net ammount

* fixes discounts incorrectly defined as tax free

* fixes global discount and coupon interaction

* disallows tax free discounts when there's no tax free items

* refactors create_json_openfactura

* fixes fees calculations

* Refactors item net calculation in shipping and tax free discount error handling

* improves admin menu responsiveness

* add gitignore

* Revert "add gitignore"

This reverts commit d323cbe.

* fixes logo url limitation and adds a save changes snackbar

* fixes broken example image in admin panel