Progress towards React 16 compatibility

[t3chguy]

• Update react-gemini-scrollbar to pull in its React 16 compatibility
• Correct the default props syntax in create-react-class
• Start transitioning from React.createClass to create-react-class

Subset of #3270

[turt2live]

it makes me slightly sad that we're going with create-react-class instead of real ES6 classes, but not enough to block this PR.

[t3chguy]

it makes me slightly sad that we're going with create-react-class instead of real ES6 classes, but not enough to block this PR.

Me too @turt2live but I know ES classes will make the PRs take a lot longer, I'd rather get everything up to React 16 and THEN transition to ES6 classes, as well as TS :D