Added manual VAT handling

> Status: on hold, see new approach in #17
> Sub-task: [EZP-23049](https://jira.ez.no/browse/EZP-23049)
> Main-task: [EZP-23034](https://jira.ez.no/browse/EZP-23034)

Adds external storage to handle manual VAT rate.

## TODO
- [ ] Squash
- [ ] Update `Price\Value` with the new properties for VAT
- [x] Fix the case where data_text is empty (demobundle data)