Solves the problem of generating invoices when placing an order.

[tomkalon]

Q	A
Branch?	2.0
Bug fix?	no
New feature?	no
BC breaks?	no
Related tickets	#127
License	MIT

It solves one problem, but it can generate others. This happens when the plugin is modified.
Example scenario:

• The customer places an order.
• An invoice record with a specific number is added.
• No PDF is generated because a PDF with that name already exists.
• The user has access to the invoice that was generated earlier. Perhaps it is someone else's invoice.

Here, this problem has been solved, but it involves a fairly radical change in structure: #398