Purchase invoice functional test

I implement a functional test case for purchase invoices including
testing the associated documents.  Doing this, I fix some issues:

- invalid arguments when calling FileService.removeFile()
- add missing document.less resource to purchase invoice form
- add missing "create" button in purchase invoice show view
- add missing "create" button to scaffolding show view
- add error messages to vendor field in purchase invoice form
- fix tab/space issues and remove parenthesis from constraints