Refactor ProtocolScreen

[wwqrd]

Can we improve the onClickNext(), onClickPrevious() methods?

Suggestions:

• Generalise the skipToIndex logic so that it can be used by both methods
• Can we make the distinction clearer between? interfaceRefInstance.clickPrevious(), promptBackward(), and this.props.changeStage(`${this.props.pathPrefix}/${skipToIndex}`);
• Can we update the logic to remove the nested if/else?
• Can we remove interfaceRefInstance? If not, can we make the distinction clearer between clickPrevious and promptBackward?