Update to flow 0.21.0

This found that we missed the `key` in `DOMEvent` as documented here:
https://facebook.github.io/react/docs/events.html#keyboard-events

Author: kassens

.flowconfig

@@ -15,4 +15,4 @@ suppress_comment=\\(.\\|\n\\)*\\$FlowFixMe
 suppress_comment=\\(.\\|\n\\)*\\$FlowIssue
 
 [version]
-0.20.1
+0.21.0

frontend/types.js

@@ -52,4 +52,5 @@ export type DOMEvent = {
   stopPropagation: () => void,
   cancelBubble: boolean,
   keyCode: number,
+  key: string,
 };

package.json

@@ -13,7 +13,7 @@
     "eslint-plugin-react": "3.12.0",
     "fbjs": "0.5.1",
     "fbjs-scripts": "0.5.0",
-    "flow-bin": "0.20.1",
+    "flow-bin": "0.21.0",
     "immutable": "3.7.6",
     "jest-cli": "0.8.2",
     "json-loader": "0.5.4",