Update React guide in code style

This updates React guidance to prefer JS classes and adds additional info about
how to handle specific situations when using them.

Signed-off-by: J. Ryan Stinnett <jryans@gmail.com>

code_style.md

@@ -165,7 +165,6 @@ ECMAScript
 
 React
 -----
-- Use React.createClass rather than ES6 classes for components, as the boilerplate is way too heavy on ES6 currently.  ES7 might improve it.
 - Pull out functions in props to the class, generally as specific event handlers:
 
   ```jsx
@@ -175,10 +174,37 @@ React
   <Foo onClick={this.onFooClick}> // Best, if onFooClick would do anything other than directly calling doStuff
   ```
 
-  Not doing so is acceptable in a single case; in function-refs:
+  Not doing so is acceptable in a single case: in function-refs:
 
   ```jsx
   <Foo ref={(self) => this.component = self}>
   ```
+
+- Prefer classes that extend `React.Component` (or `React.PureComponent`) instead of `React.createClass`
+  - You can avoid the need to bind handler functions by using [property initializers](https://reactjs.org/docs/react-component.html#constructor):
+
+  ```js
+  class Widget extends React.Component
+      onFooClick = () => {
+          ...
+      }
+  }
+  ```
+  - To define `propTypes`, use a static property:
+  ```js
+  class Widget extends React.Component
+      static propTypes = {
+          ...
+      }
+  }
+  ```
+  - If you need to specify initial component state, [assign it](https://reactjs.org/docs/react-component.html#constructor) to `this.state` in the constructor:
+  ```js
+  constructor(props) {
+    super(props);
+    // Don't call this.setState() here!
+    this.state = { counter: 0 };
+  }
+  ```
 - Think about whether your component really needs state: are you duplicating
   information in component state that could be derived from the model?