Backport PDF when submissions need to be cosigned

[sergei-maertens]

Requested by DH as they're worried about legal implications of the language used currently.

Taken from #4320 / #4838

[sergei-maertens]

@vaszig this needs to go in the 2.8.2 patch release :)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.56%. Comparing base (fa41122) to head (8cf6f6e).

Files with missing lines	Patch %	Lines
src/openforms/submissions/models/submission.py	60.00%	1 Missing and 1 partial :warning:
src/openforms/submissions/report.py	60.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stable/2.8.x #4846 +/- ## ================================================ - Coverage 96.58% 96.56% -0.02% ================================================ Files 746 746 Lines 25247 25256 +9 Branches 3325 3328 +3 ================================================ + Hits 24384 24389 +5 - Misses 601 603 +2 - Partials 262 264 +2 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles