Adds specific tax rates to each type of cost

[filipefurtad0]

What? Why?

Closes #9556

It adds three different tax rates to:

• the enterprise fee (15%)
• the shipping fee (10%)
• the line items (20%)

It also changes the way the enterprise fee is set up:

• let! is used instead of let
• it explicitly references its own tax category; before it was fetching the tax category from a product

What should we test?

Consistently green build.

Release notes

Changelog Category: Technical changes

The title of the pull request will be included in the release notes.

[mkllnk]

Okay, this is great for now and I'll merge it.

@RachL comment made me think though that we need to add another spec with two tax rates that have the same percentage (20%) as this spec was before.

In FR alcohol is taxed at 20% but so is shipping.

I think that this bug affects French invoices, displaying only the alcohol part or the shipping part in the total if both are present.