Added disabled property to send button

[tdurnford]

Fixes #1920

Added the disabled property to the send button, so the 'sendBoxButtonColorOnDisabled' default style applies to the button when Web Chat is disabled.

Example Code

  render() {

    const styleOptions = { 
      sendBoxButtonColorOnDisabled: 'blue'
    }
    
    return (
      this.state.directLine
        ? <ReactWebChat
          className="chat"
          disabled={true}
          directLine={ this.state.directLine }
          styleOptions={ styleOptions }
          { ...this.props }
        />
        :<div>Connecting to bot&hellip;</div>
    );
  }

Screenshot

[coveralls]

Coverage remained the same at 54.05% when pulling 308d484 on tdurnford:1920 into 9cbbea2 on Microsoft:master.

[corinagum]

LGTM