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/common/test/directives/ng_component_outlet_spec.ts

@@ -18,7 +18,7 @@ export function main() {
     beforeEach(() => { TestBed.configureTestingModule({imports: [TestModule]}); });
 
     it('should do nothing if component is null', async(() => {
-         const template = `<template *ngComponentOutlet="currentComponent"></template>`;
+         const template = `<ng-template *ngComponentOutlet="currentComponent"></ng-template>`;
          TestBed.overrideComponent(TestComponent, {set: {template: template}});
          let fixture = TestBed.createComponent(TestComponent);
 
@@ -120,7 +120,7 @@ export function main() {
        }));
 
     it('should render projectable nodes, if supplied', async(() => {
-         const template = `<template>projected foo</template>${TEST_CMP_TEMPLATE}`;
+         const template = `<ng-template>projected foo</ng-template>${TEST_CMP_TEMPLATE}`;
          TestBed.overrideComponent(TestComponent, {set: {template: template}})
              .configureTestingModule({schemas: [NO_ERRORS_SCHEMA]});
 
@@ -221,7 +221,7 @@ class InjectedComponentAgain {
 }
 
 const TEST_CMP_TEMPLATE =
-    `<template *ngComponentOutlet="currentComponent; injector: injector; content: projectables; ngModuleFactory: module;"></template>`;
+    `<ng-template *ngComponentOutlet="currentComponent; injector: injector; content: projectables; ngModuleFactory: module;"></ng-template>`;
 @Component({selector: 'test-cmp', template: TEST_CMP_TEMPLATE})
 class TestComponent {
   currentComponent: Type<any>;