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/core/src/linker/template_ref.ts

@@ -14,10 +14,10 @@ import {EmbeddedViewRef} from './view_ref';
 /**
  * Represents an Embedded Template that can be used to instantiate Embedded Views.
  *
- * You can access a `TemplateRef`, in two ways. Via a directive placed on a `<template>` element (or
- * directive prefixed with `*`) and have the `TemplateRef` for this Embedded View injected into the
- * constructor of the directive using the `TemplateRef` Token. Alternatively you can query for the
- * `TemplateRef` from a Component or a Directive via {@link Query}.
+ * You can access a `TemplateRef`, in two ways. Via a directive placed on a `<ng-template>` element
+ * (or directive prefixed with `*`) and have the `TemplateRef` for this Embedded View injected into
+ * the constructor of the directive using the `TemplateRef` Token. Alternatively you can query for
+ * the `TemplateRef` from a Component or a Directive via {@link Query}.
  *
  * To instantiate Embedded Views based on a Template, use
  * {@link ViewContainerRef#createEmbeddedView}, which will create the View and attach it to the

modules/@angular/core/src/linker/view_ref.ts

@@ -57,7 +57,7 @@ export abstract class ViewRef extends ChangeDetectorRef {
  * ```
  * Count: {{items.length}}
  * <ul>
- *   <template ngFor let-item [ngForOf]="items"></template>
+ *   <ng-template ngFor let-item [ngForOf]="items"></ng-template>
  * </ul>
  * ```
  *
@@ -74,7 +74,7 @@ export abstract class ViewRef extends ChangeDetectorRef {
  * <!-- ViewRef: outer-0 -->
  * Count: 2
  * <ul>
- *   <template view-container-ref></template>
+ *   <ng-template view-container-ref></ng-template>
  *   <!-- ViewRef: inner-1 --><li>first</li><!-- /ViewRef: inner-1 -->
  *   <!-- ViewRef: inner-2 --><li>second</li><!-- /ViewRef: inner-2 -->
  * </ul>

modules/@angular/core/src/linker/view_type.ts

@@ -10,10 +10,8 @@ export enum ViewType {
   // A view that contains the host element with bound component directive.
   // Contains a COMPONENT view
   HOST,
-  // The view of the component
-  // Can contain 0 to n EMBEDDED views
+  // The view of the component can contain 0 to n EMBEDDED views
   COMPONENT,
-  // A view that is embedded into another View via a <template> element
-  // inside of a COMPONENT view
+  // A view is embedded into another View via a <ng-template> element inside of a COMPONENT view
   EMBEDDED
 }