feat(compiler): introduce <ng-template>, deprecate <template> and template attribute

The rationale of this change is to improve the inter-operability with web
components that might make use of the `<template>` tag.

DEPRECATION

The template tags and template attribute are deprecated:

    <template ngFor [ngFor]=items let-item><li>...</li></template>
    <li template="ngFor: let item of items">...</li>

should be rewritten as:

    <ng-template ngFor [ngFor]=items let-item><li>...</li></ng-template>

Note that they still be supported in 4.x with a deprecartion warning in
development mode.

MIGRATION

- `template` tags (or elements with a `template` attribute) should be rewritten
as a `ng-template` tag,
- `ng-content` selectors should be updated to referto a `ng-template` where they
use to refer to a template: `<ng-content selector="template[attr]">` should be
rewritten as `<ng-content selector="ng-template[attr]">`
- if you consume a component relying on your templates being actual `template`
elements (that is they include a `<ng-content selector="template[attr]">`). You
should  still migrate to `ng-template` and make use of `ngProjectAs` to override
the way `ng-content` sees the template:
`<ng-template projectAs="template[attr]">`
- while `template` elements are deprecated in 4.x they continue to work.

modules/@angular/compiler/src/jit/compiler_factory.ts

@@ -109,12 +109,15 @@ export const COMPILER_PROVIDERS: Array<any|Type<any>|{[k: string]: any}|any[]> =
 export class JitCompilerFactory implements CompilerFactory {
   private _defaultOptions: CompilerOptions[];
   constructor(@Inject(COMPILER_OPTIONS) defaultOptions: CompilerOptions[]) {
-    this._defaultOptions = [<CompilerOptions>{
-                             useDebug: isDevMode(),
-                             useJit: true,
-                             defaultEncapsulation: ViewEncapsulation.Emulated,
-                             missingTranslation: MissingTranslationStrategy.Warning,
-                           }].concat(defaultOptions);
+    const compilerOptions: CompilerOptions = {
+      useDebug: isDevMode(),
+      useJit: true,
+      defaultEncapsulation: ViewEncapsulation.Emulated,
+      missingTranslation: MissingTranslationStrategy.Warning,
+      enableLegacyTemplate: true,
+    };
+
+    this._defaultOptions = [compilerOptions, ...defaultOptions];
   }
   createCompiler(options: CompilerOptions[] = []): Compiler {
     const opts = _mergeOptions(this._defaultOptions.concat(options));