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_template_outlet_spec.ts

@@ -41,14 +41,14 @@ export function main() {
        }));
 
     it('should insert content specified by TemplateRef', async(() => {
-         const template = `<template #tpl>foo</template>` +
+         const template = `<ng-template #tpl>foo</ng-template>` +
              `<ng-container [ngTemplateOutlet]="tpl"></ng-container>`;
          fixture = createTestComponent(template);
          detectChangesAndExpectText('foo');
        }));
 
     it('should clear content if TemplateRef becomes `null`', async(() => {
-         const template = `<tpl-refs #refs="tplRefs"><template>foo</template></tpl-refs>` +
+         const template = `<tpl-refs #refs="tplRefs"><ng-template>foo</ng-template></tpl-refs>` +
              `<ng-container [ngTemplateOutlet]="currentTplRef"></ng-container>`;
          fixture = createTestComponent(template);
          fixture.detectChanges();
@@ -63,7 +63,7 @@ export function main() {
 
     it('should swap content if TemplateRef changes', async(() => {
          const template =
-             `<tpl-refs #refs="tplRefs"><template>foo</template><template>bar</template></tpl-refs>` +
+             `<tpl-refs #refs="tplRefs"><ng-template>foo</ng-template><ng-template>bar</ng-template></tpl-refs>` +
              `<ng-container [ngTemplateOutlet]="currentTplRef"></ng-container>`;
          fixture = createTestComponent(template);
 
@@ -78,14 +78,14 @@ export function main() {
        }));
 
     it('should display template if context is `null`', async(() => {
-         const template = `<template #tpl>foo</template>` +
+         const template = `<ng-template #tpl>foo</ng-template>` +
              `<ng-container *ngTemplateOutlet="tpl; context: null"></ng-container>`;
          fixture = createTestComponent(template);
          detectChangesAndExpectText('foo');
        }));
 
     it('should reflect initial context and changes', async(() => {
-         const template = `<template let-foo="foo" #tpl>{{foo}}</template>` +
+         const template = `<ng-template let-foo="foo" #tpl>{{foo}}</ng-template>` +
              `<ng-container *ngTemplateOutlet="tpl; context: context"></ng-container>`;
          fixture = createTestComponent(template);
 
@@ -97,7 +97,7 @@ export function main() {
        }));
 
     it('should reflect user defined `$implicit` property in the context', async(() => {
-         const template = `<template let-ctx #tpl>{{ctx.foo}}</template>` +
+         const template = `<ng-template let-ctx #tpl>{{ctx.foo}}</ng-template>` +
              `<ng-container *ngTemplateOutlet="tpl; context: context"></ng-container>`;
          fixture = createTestComponent(template);
          fixture.componentInstance.context = {$implicit: {foo: 'bra'}};
@@ -105,7 +105,8 @@ export function main() {
        }));
 
     it('should reflect context re-binding', async(() => {
-         const template = `<template let-shawshank="shawshank" #tpl>{{shawshank}}</template>` +
+         const template =
+             `<ng-template let-shawshank="shawshank" #tpl>{{shawshank}}</ng-template>` +
              `<ng-container *ngTemplateOutlet="tpl; context: context"></ng-container>`;
          fixture = createTestComponent(template);