PR: Add preferred dir option to config

[goanpeca]

Following on the discussion of adding a new trait preferred-dir

And the PR on JServer -> jupyter-server/jupyter_server#549

This PR now exposes that option on the page config. I added both the full url and the relative path to use if found in the frontend which is using it over at jupyterlab/jupyterlab#10667

[codecov-commenter]

Codecov Report

Merging #190 (17e17d7) into master (3c7a83c) will decrease coverage by 0.12%.
The diff coverage is 85.71%.

❗ Current head 17e17d7 differs from pull request most recent head 894e0da. Consider uploading reports for the commit 894e0da to get more accurate results

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   78.70%   78.58%   -0.13%     
==========================================
  Files          27       27              
  Lines        2010     2003       -7     
==========================================
- Hits         1582     1574       -8     
- Misses        428      429       +1     

Impacted Files	Coverage Δ
jupyterlab_server/handlers.py	89.10% <85.71%> (-0.16%)	⬇️
jupyterlab_server/settings_handler.py	85.18% <0.00%> (-0.27%)	⬇️
jupyterlab_server/tests/test_settings_api.py	100.00% <0.00%> (ø)

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[blink1073]

Thanks!