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/src/directives/ng_for_of.ts

@@ -70,12 +70,12 @@ export class NgForOfRow<T> {
  * - `<li *ngFor="let item of items; let i = index; trackBy: trackByFn">...</li>`
  * - `<li template="ngFor let item of items; let i = index; trackBy: trackByFn">...</li>`
  *
- * With `<template>` element:
+ * With `<ng-template>` element:
  *
  * ```
- * <template ngFor let-item [ngForOf]="items" let-i="index" [ngForTrackBy]="trackByFn">
+ * <ng-template ngFor let-item [ngForOf]="items" let-i="index" [ngForTrackBy]="trackByFn">
  *   <li>...</li>
- * </template>
+ * </ng-template>
  * ```
  *
  * ### Example

modules/@angular/common/src/directives/ng_if.ts

@@ -26,7 +26,7 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef} from '
  * # Showing an alternative template using `else`
  *
  * If it is necessary to display a template when the `expression` is falsy use the `else` template
- * binding as shown. Note that the `else` binding points to a `<template>` labeled `#elseBlock`.
+ * binding as shown. Note that the `else` binding points to a `<ng-template>` labeled `#elseBlock`.
  * The template can be defined anywhere in the component view but is typically placed right after
  * `ngIf` for readability.
  *
@@ -76,25 +76,25 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef} from '
  * Simple form:
  * - `<div *ngIf="condition">...</div>`
  * - `<div template="ngIf condition">...</div>`
- * - `<template [ngIf]="condition"><div>...</div></template>`
+ * - `<ng-template [ngIf]="condition"><div>...</div></ng-template>`
  *
  * Form with an else block:
  * ```
  * <div *ngIf="condition; else elseBlock">...</div>
- * <template #elseBlock>...</template>
+ * <ng-template #elseBlock>...</ng-template>
  * ```
  *
  * Form with a `then` and `else` block:
  * ```
  * <div *ngIf="condition; then thenBlock else elseBlock"></div>
- * <template #thenBlock>...</template>
- * <template #elseBlock>...</template>
+ * <ng-template #thenBlock>...</ng-template>
+ * <ng-template #elseBlock>...</ng-template>
  * ```
  *
  * Form with storing the value locally:
  * ```
  * <div *ngIf="condition; else elseBlock; let value">{{value}}</div>
- * <template #elseBlock>...</template>
+ * <ng-template #elseBlock>...</ng-template>
  * ```
  *
  * @stable