Ability to mark preview generated PDF's with "not valid"

[RonnyB71]

Consider if this should be a part of the watermark issue as one option would be to show the "not valid" as a watermark.

The main requirement of this issue is to clearly show that the PDF is a preview and not based on submitted data. The rationale behind this is that service owners have seen abuse of generated PDF as if they have been submitted. Although, there is no guarantee for misuse as anyone with a PDF tool for editing PDFs would be able to remove such a mark, but it would heighten the bar for misuse.

Acceptance criterias

• Clearly mark a preview PDF with a "not valid" mark (should be configurable string value)
• Should show the mark regardless if you use the native browser print version of a page or our preview pdf

[RonnyB71]

Closing this issue as it will by solved by Altinn/app-lib-dotnet#787