Add hide ScrollToEnd button through style options

[nt-7]

Add option to hide scroll button #2307

Description

This work will enable to hide the ScrollToEndButton ("New messages") through style options from webchat UI when scrolling.

Example

As below, hideScrollToEndButton on styleOptions enable users to hide the ScrollToEndButton from webchat UI.

const styleOptions = {
    hideScrollToEndButton: false
};
window.WebChat.renderWebChat({
    directLine: window.WebChat.createDirectLine({ token }),
    styleOptions
}, document.getElementById('webchat'));

[coveralls]

Coverage increased (+0.02%) to 66.13% when pulling 0400213 on nt-7:hide-scroll-button into 007d5dc on microsoft:master.

[tdurnford]

Thanks for your contribution! Would you mind adding a CHANGELOG entry to your PR?

[nt-7]

@tdurnford thanks for your response! I updated CHANGELOG now.

[corinagum]

Thank you for your contribution! :)