Clean up and simplify WebSocket implementation on the JS side

Summary:- Get rid of no longer necessary WebSocket.js v WebSocketBase.js split
- Use `EventTarget(list, of, events)` as base class to auto-generate `oneventname` getters/setters that get invoked along with other event handlers
- Type annotation `any` considered harmful, especially when we can easily spell out the actual type
- Throw in some `const` goodness for free

**Test Plan:** Launch UIExplorer example app, supplied `websocket_test_server` script, and try different combinations of sending and receiving text and binary data on both iOS and Android.
Closes https://github.com/facebook/react-native/pull/6889

Differential Revision: D3184835

Pulled By: mkonicek

fb-gh-sync-id: f21707f4e97aa5a79847f5157e0a9f132a1a01cd
fbshipit-source-id: f21707f4e97aa5a79847f5157e0a9f132a1a01cd

Examples/UIExplorer/UIExplorerList.android.js

@@ -174,6 +174,10 @@ const APIExamples = [
     key: 'VibrationExample',
     module: require('./VibrationExample'),
   },
+  {
+    key: 'WebSocketExample',
+    module: require('./WebSocketExample'),
+  },
   {
     key: 'XHRExample',
     module: require('./XHRExample'),