WIP - Allow notebooks to be trusted

[mwouts]

This PR relates to #797 and to the logs 'notebook is not trusted'.

Here we provide a test that reproduces what I have seen in a debug session, i.e. that notebooks created in Jupyter Lab might not have cell ids when they are passed to cm.save, and a workaround - but the workaround might have to go to the core contents manager.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@94ac59d). Click here to learn what that means.
The diff coverage is 97.55%.

@@            Coverage Diff            @@
##             master     #825   +/-   ##
=========================================
  Coverage          ?   99.10%           
=========================================
  Files             ?      106           
  Lines             ?    10330           
  Branches          ?        0           
=========================================
  Hits              ?    10238           
  Misses            ?       92           
  Partials          ?        0           

Impacted Files	Coverage Δ
jupytext/reraise.py	25.00% <25.00%> (ø)
jupytext/kernels.py	92.85% <92.85%> (ø)
jupytext/pep8.py	95.58% <95.58%> (ø)
jupytext/cli.py	96.12% <96.07%> (ø)
jupytext/cell_reader.py	96.86% <96.49%> (ø)
jupytext/pandoc.py	96.61% <96.61%> (ø)
jupytext/cell_to_text.py	97.80% <97.16%> (ø)
jupytext/myst.py	97.19% <97.19%> (ø)
jupytext/config.py	97.28% <97.28%> (ø)
jupytext/contentsmanager.py	97.87% <97.75%> (ø)
... and 17 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 94ac59d...378fe1b. Read the comment docs.

[mwouts]

I've taken #826 instead