feat(security): add an HTML sanitizer.

This is based on Angular 1's implementation, parsing an HTML document
into an inert DOM Document implementation, and then serializing only
specifically whitelisted elements.

It currently does not support SVG sanitization, all SVG elements are
rejected.

If available, the sanitizer uses the `<template>` HTML element as an
inert container.

Sanitization works client and server-side.

Reviewers: rjamet, tbosch , molnarg , koto

Differential Revision: https://reviews.angular.io/D108

modules/@angular/platform-browser/src/browser/browser_adapter.ts

@@ -125,6 +125,9 @@ export class BrowserDomAdapter extends GenericBrowserDomAdapter {
     return evt.defaultPrevented || isPresent(evt.returnValue) && !evt.returnValue;
   }
   getInnerHTML(el): string { return el.innerHTML; }
+  getTemplateContent(el): Node {
+    return 'content' in el && el instanceof HTMLTemplateElement ? el.content : null;
+  }
   getOuterHTML(el): string { return el.outerHTML; }
   nodeName(node: Node): string { return node.nodeName; }
   nodeValue(node: Node): string { return node.nodeValue; }