[explorev2] improving the scrolling/scrollbars placement

[mistercrunch]

For the controls, moving the scrollbar outside the panel as opposed to inside the panel.
Before:

After:

Also, in v1, the chart container doesn't allow to overflow-x, unless the viz's CSS override that. Matching this behavior here so that the table view doesn't show an X axis scrollbar.

[ascott]

this was added because there are some charts that don't scale to the width of the container, and so a scrollbar was needed to see the rest of the chart. i thought i did see this functionality in v1. i will try and find an example.

[mistercrunch]

I did a bit of code archeology for this as I remembered fixing the call. overflow-x was introduced here 2d0ebea, but wasn't valid React style, so I fixed the syntax here c155857 , but by fixing it I really just brought something we didn't want.

Here are example of viz that override to overflow-x to set it to auto:
https://github.com/airbnb/superset/blob/master/superset/assets/visualizations/nvd3_vis.css#L15
https://github.com/airbnb/superset/blob/master/superset/assets/visualizations/nvd3_vis.css#L28

[mistercrunch]

Somehow setting overflow-x to auto makes the table and pivot table viz show too many scrollbars because of the way the "fixed header" scrolling is implemented

[ascott]

i'm ok with taking this out. i'll add an issue to investigate how viz's are scaled width wise, to make sure there aren't cases where part of the viz is cut off.