Remove $el_to_style from the widget javascript code; '' now defaults to this.$el

This seems to make it easiest to select the top-level element.  An alternative is to use special syntax for top-level elements, like in https://github.com/ipython/ipython/pull/6185

This will be followed up by other fixes that let the user set specific common properties of different elements.

This change also adds the .addBack() call, which means that nonempty selectors now apply to this.$el and its descendants, rather than just the descendants of this.$el.
Fixed pandoc version detection.
Fix js-tests for new default selector logic.
Merge pull request #5 from jdfreder/css-top-default

Fix js-tests for new default selector logic.
avoid unregistering widget model twice

widgets are registered when the comm is created;
closing unregisters them. Calling `close` a second time
should be a no-op.
Revised pandoc version detection.
Fixed indentation error.
Allow either okay or accept_role to be returned by confirm exit.
Merge pull request #6261 from blink1073/master

Allow either okay or accept_role to be returned by QtConsole confirm exit.
Merge pull request #6250 from minrk/double-free-widget

avoid unregistering widget model twice
Merge pull request #6226 from jasongrout/css-top-default

Remove $el_to_style from the widget javascript code; '' now defaults to this.$el
Upgrade default mathjax version.

This also fixes an issue with ```from IPython.external import mathjax; mathjax.install_mathjax()``` not working properly, because of a changed version schema.
Suppress virtualenv warning messages on Windows

Suppress 'WARNING: Attempting to work in a virtualenv...' message
on Windows in bash and PowerShell.  Adapts
pip.locations.running_under_virtualenv for use in no sys.executable
symlinks case.
Apply os.path.normcase to venv detection
Add local variable for normalized virtualenv path
Merge pull request #6020 from etgalloway/bash-windows-venv-warning

Disable virtualenv warning in bash on Windows
Merge pull request #6241 from cel4/pandoc_fix

Fixed pandoc version detection.
Merge pull request #6273 from bgruening/patch-1

Upgrade default mathjax version.