account_mtid_vat v8.0 inheritance chain broken

[luc-demeyer]

no super on _sum, _sum_year, _sum_period in account_tax_code
no super on active in view_tree_js

This is no issue for my current implementation of the mtd module but may cause problems in multi-company/multi-country environments.

[Nick-OpusVL]

Note that the override of _sum no longer exists and there's a question mark (#10) over the continued existence of _sum_year.

I agree though that the others need consideration, but it will need some thought to work out how and under what conditions we will fall back to the core behaviour by calling super().