New config option cm_config_log_level

[mwouts]

The default value for the new option is 'info_if_changed' to reduce the number of lines logged by the contents manager.
Closes #959

[mwouts]

We'll have to fix #964 first...

[codecov]

Codecov Report

Merging #963 (5d9a609) into main (112c6a3) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #963      +/-   ##
==========================================
- Coverage   99.07%   99.05%   -0.02%     
==========================================
  Files         117      117              
  Lines       10898    10956      +58     
==========================================
+ Hits        10797    10853      +56     
- Misses        101      103       +2     

Impacted Files	Coverage Δ
jupytext/config.py	97.50% <100.00%> (+0.07%)	⬆️
jupytext/contentsmanager.py	95.83% <100.00%> (-0.55%)	⬇️
tests/conftest.py	100.00% <100.00%> (ø)
tests/test_cm_config.py	100.00% <100.00%> (ø)

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