updated Admin to use ES6 classes
updated Admin to use ES6 classes

React.createClass is deprecated in React 15.5.0. Convert Admin to use 
regular ES6 classes, following [this 
post](https://daveceddia.com/convert-createclass-to-es6-class/).

mjec/rc-niceties,mjec/rc-niceties,mjec/rc-niceties