account_financial_report_webkit_xls: Port to 8.0

I ported the module "account_financial_report_webkit_xls" to 8.0.

This pull request is based on https://github.com/OCA/reporting-engine/pull/5.

All the modifications in this pull request are the result of the migration of account_financial_report_webkit to v8 (see https://github.com/OCA/account-financial-reporting/pull/20).