Add chart export module

[bzkrouse]

Re-implement chart export module #449.

Further discussion/to-dos:

• Consider workflow for replicating charts with R code
  • start of some helpers are embedded but we should think about making this as user friendly as possible
• Modify/extract helper functions to /R directory
• Reuse helpers in chart creation modules

[jwildfire]

Pretty sure we need to add the init function (if any) in to the workflow here. I'll see what I can do in the next few days.

[jwildfire]

Widgets aren't rendering for me in the reports - I suspect it's because the init functions aren't running as expected. Going to try setting it up so that params are defined using the exact same process as here.

[jwildfire]

Pushed a tweak that helps deal with the init, but there are still a few issues:

• hep-explorer and shiftplot aren't working because they use x.ns (instead of the native el) from the widget settings to identify the div to draw the chart. Probably need to update the renderers, but I'm pretty sure this was a hack to deal with rendering issues in the app. Filed Don't use namespace when rendering widgets safetyCharts#49.
• the shifttable with Kable is bombing both here and in the app. Leaning towards deleting. Filed tpylr shift table isn't rendering safetyCharts#50
• ae-timeline bombs when aeexplorer is present, but renderers fine otherwise. Guessing it just wants to be drawn first. Errors in js, make it seem like the linked table isn't rendering properly ...

Everything is also a bit slow, so probably need some kind of load bar.

[jwildfire]

Merging - thanks for setting this up @bzkrouse!

Filed #518, #519 and #520 in this repo and SafetyGraphics/safetyCharts#49 and SafetyGraphics/safetyCharts#50 in safetyCharts to deal with down the road.

[jwildfire]

👍