set default config dir name

[minrk]

default is not a kwarg of the traitlet constructor (default_value is)

The result was a warning on every jupyter server startup that tag metadata was being stored, but this is deprecated. The default value was not being set.

[codecov-commenter]

Codecov Report

Merging #504 (1c00e8c) into master (5e77b73) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
- Coverage   77.68%   77.64%   -0.05%     
==========================================
  Files         106      106              
  Lines        9276     9276              
  Branches     1001     1001              
==========================================
- Hits         7206     7202       -4     
- Misses       1711     1713       +2     
- Partials      359      361       +2     

Impacted Files	Coverage Δ
jupyter_server/services/config/manager.py	100.00% <ø> (ø)
jupyter_server/services/contents/filemanager.py	65.48% <0.00%> (-0.84%)	⬇️
jupyter_server/serverapp.py	67.02% <0.00%> (-0.22%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	81.09% <0.00%> (+0.72%)	⬆️

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[minrk]

It's a bit unclear if this is the right thing to do, vs remove the setting that doesn't do anything - this changes the default config path to $config_path/serverconfig instead of the current (unintended, I think) $config_path itself. This might break things, but I'm not 100% sure exactly when the default is used. Server extensions, for instance, I believe explicitly set the secondary properties (read/write_config_path), meaning they shouldn't be affected.

However, the REST API for server-side config manager should be affected by this, I think.

[Zsailer]

For reference, this is hardcoded in jupyter/notebook: https://github.com/jupyter/notebook/blob/8ca28879202296176d8d9b6ba76704462d1dcd1b/notebook/services/config/manager.py#L48