Time based PDF upload warning

[felix-engelmann]

The warning messages on the paper edit page are very helpful in hinting to wrongly filled fields. An enhancement could be a check on the uploaded PDF to verify that the freshly uploaded PDF has a newer creation time stamp that the one present.
This would catch accidental uploads of wrong PDFs as an update should use a newer PDF.

I looked into creating a PR, but I'm don't understand the architecture well enough to decide where a good place for such a feature would be. The banal script does not extract this information and doing so would need a dependency, e.g. https://metacpan.org/pod/PDF::API2 . However similarly extracting the information in PHP requires some dependency.

I saw that the banal data is cached in the PaperStorage table which would allow a comparison to the creation timestamp of the previous upload.
In what I saw, checkformat.php does not care about old versions. So I'm not sure if a creation time check should be implemented like this.

What is a good path to implement this feature? Add a perl dependency or proceed in php?