Implement sliders properly

[forsyth2]

Since we are disabling sliders in Pull Request #426, we would like to eventually enable them again once we have fixed their associated bugs.

One bug is Issue #424 (plots not displaying when downloaded as html). Another bug is that sliders replicate after each use.

We know that using VBox instead of IPythonDisplay caused the bug in Issue #424. A couple questions arise from this:

• jupyter nbconvert --execute --to html <notebook> will display the plot in html, but running the notebook first and then a) jupyter nbconvert --to html <notebook> or b) "File" > "Download as" > "HTML (.html)" will not display the plot. Why would using nbconvert --execute yield a different result than running the notebook from the web? This could be a bug in Jupyter that should be reported.
• If running the notebook from the web is in fact different from using nbconvert --execute then what is going wrong? a) Does Jupyter have a bug where if a notebook is run from the web server then the plots won't download? b) Does VBox (or perhaps ipywidgets in general) have a bug where it doesn't download properly if a notebook is run from the web server? c) Are we using VBox wrong?