Add scroll additions

[zoesteinkamp]

Closes #476

Changes

• Expose onScroll and onUpdate props in DapperScrollbars from react-scrollbars-custom library
• Add story with 2 instances of DapperScrollbars that have synchronized scrolling
  • This example is a way to ensure the required functionality in InfluxDB can be built locally
• Create types for the events returned to onScroll and onUpdate as well as the handler functions
  • This was challenging as the typing within the library is highly unusual and is not exported

Screenshots

Checklist

Check all that apply

• Updated documentation to reflect changes
• Added entry to top of Changelog with link to PR (not issue)
• Tests pass
• Peer reviewed and approved
• Signed CLA (if not already signed)