Merge overrides settings if values are dicts

[goanpeca]

Fixes jupyterlab/jupyterlab#10010

When using overrides files, if a default value in a schema is a dict then it will update the default with the dict provided by the overrides file.

With overrides.json

{
  "@jupyterlab/notebook-extension:tracker": {
      "codeCellConfig": {
         "lineNumbers": false
      }
  }
}

Before this change

The resulting system default settings would be:

{
    // Notebook
    // @jupyterlab/notebook-extension:tracker
    // Notebook settings.
    // **************************************

    // Code Cell Configuration
    // The configuration for all code cells.
    "codeCellConfig": {
        "lineNumbers": false,
    },
....

After this change

The resulting system default settings would be:

{
    // Notebook
    // @jupyterlab/notebook-extension:tracker
    // Notebook settings.
    // **************************************

    // Code Cell Configuration
    // The configuration for all code cells.
    "codeCellConfig": {
        "autoClosingBrackets": true,
        "cursorBlinkRate": 530,
        "fontFamily": null,
        "fontSize": null,
        "lineHeight": null,
        "lineNumbers": false,
        "lineWrap": "off",
        "matchBrackets": true,
        "readOnly": false,
        "insertSpaces": true,
        "tabSize": 4,
        "wordWrapColumn": 80,
        "rulers": [],
        "codeFolding": false,
        "lineWiseCopyCut": true
    },
....

[codecov-commenter]

Codecov Report

Merging #188 (570e2b5) into master (6ecc630) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   78.55%   78.70%   +0.14%     
==========================================
  Files          27       27              
  Lines        1996     2010      +14     
==========================================
+ Hits         1568     1582      +14     
  Misses        428      428              

Impacted Files	Coverage Δ
jupyterlab_server/settings_handler.py	85.45% <100.00%> (+0.26%)	⬆️
jupyterlab_server/tests/test_settings_api.py	100.00% <100.00%> (ø)

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[goanpeca]

The errors seem unrelated to the changes on this PR.

+ make html
Running Sphinx v4.0.3

Exception occurred:
  File "/usr/local/miniconda/envs/jupyterlab_server_documentation/lib/python3.8/site-packages/autodoc_traits/autodoc_traits.py", line 2, in <module>
    from sphinx.domains.python import PyClassmember
ImportError: cannot import name 'PyClassmember' from 'sphinx.domains.python' (/usr/local/miniconda/envs/jupyterlab_server_documentation/lib/python3.8/site-packages/sphinx/domains/python.py)
The full traceback has been saved in /var/folders/24/8k48jl6d249_n_qfxwsl6xvm0000gn/T/sphinx-err-zqiolyd7.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make: *** [html] Error 2

[blink1073]

Thank you!