The state of the JS documentation was quite sad, not updated in a long
time.  With this commit, we rework a lot of the JS doc.  Most notably,
we add a section 'Javascript Cheatsheet' (help on some common
customization tasks) and rewrite the 'Javascript Reference' totally.

Note that the JS reference page also contains a reference on all field
widgets.

This work is just the beginning, it is probably not complete, but at
least we will have a foundation.

Closes #20726

* Fix a bunch of ill-documented/incomplete/incorrect method docs
* add start of Sphinx extension to extract & integrate jsdoc into
  Sphinx documentation:

  - parse JS files (and don't blow up), uses a fork of pyjsparser as
    the project currently does not parse comments
  - extract cross-module dependency information
  - parse JsDoc comments using pyjsdoc and infer structure from code &
    jsdoc
  - ``ast`` CLI printing a simplified AST of the input files
  - ``dependencies`` creating a dependency graph of either all modules
    in the provided input files or the modules matching the specified
    filters (warning: will not work if missing dependencies),
    generates a .dot file
  - ``extractor`` generating a plain text module documentation (mix of
    rst and markdown styles, not anything formal)
* sphinx extension with an "automodule" directive taking a module name
  and generating the documentation for it

For consistency, every static test folder should be named 'tests', with
a s.

To avoid confusion such as in #12884

Otherwise the text is considered as a quote

The `unittest2` package is simply a backport of `unittest` from the
Standard Library of Python 2.7 to previous versions.

There is no reason to use it any longer.

Closes #6941