Include Company Tax Number

[kochie]

Hello! Long time no see, this is some code I had in a local branch that I thought I contributed back.

This PR adds Tax numbers to external sales contacts and vendors. This allows invoices to include a Tax number (like ABN for Australia) which is a requirement in some jurisdictions.

This PR closes #374

It's not quite complete yet, but it's close.

Todo

• Normalise the name so it works across regions.
• Verify the company model is valid. (I've just taken a guess)
• Feedback/Tests
• Fix Merge issues.
• Remove irrelevant changes.
• Include as option in invoice template.

I apologise in advance but I might not have enough time to see this through so if anyone wants to contribute please feel free, or just ping me.