Sidebar, Widget & Container UX Improvements

[jmuharsky]

• Reduced padding inside containers, increased padding inside widgets.
• Added padding to the sidebar panels.
• Added top margin and emphasis to sidebar group titles.
• Changed the default sidebar width from 200px to 25% of window width.
• Increased the default height of text areas (markdown) in the sidebar.
• Move Chart Config to be directly after Widget Config (before Filter/Results/Columns)

This is deployed at https://27-2-dot-perfkit-explorer.appspot.com

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