Hide scrollbars until they're needed

[bonanitech]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

As I mentioned on Slack, most sidebar panels show scrollbars even when they're not needed. This PR changes that by hiding the scrollbars until the content overflows.

Tested on a Mac in Chrome, Safari, and Firefox.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality

[coveralls]

Coverage remained the same at 68.199% when pulling f454c29 on bonanitech:overflow-auto into dc7fef6 on node-red:master.