Allow non-empty directory deletion through settings

[fcollonval]

Fixes #570

[codecov-commenter]

Codecov Report

Merging #574 (6decf7a) into master (90f619c) will increase coverage by 0.19%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #574      +/-   ##
==========================================
+ Coverage   77.12%   77.32%   +0.19%     
==========================================
  Files         109      109              
  Lines       10115    10137      +22     
  Branches     1089     1093       +4     
==========================================
+ Hits         7801     7838      +37     
+ Misses       1929     1909      -20     
- Partials      385      390       +5     

Impacted Files	Coverage Δ
jupyter_server/services/contents/filemanager.py	70.55% <71.42%> (+3.54%)	⬆️
...ter_server/tests/services/contents/test_manager.py	96.95% <90.47%> (-0.34%)	⬇️

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[blink1073]

FYI this now has a merge conflict from the formatting changes in #575

[fcollonval]

@blink1073 CI tests failing are identical to the one on master. Is this one good to go?

[blink1073]

LGTM, thanks! Let's wait for @Zsailer to have a look as well