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/examples/common/ngIf/ts/module.ts

@@ -34,7 +34,7 @@ class NgIfSimple {
     show = {{show}}
     <br>
     <div *ngIf="show; else elseBlock">Text to show</div>
-    <template #elseBlock>Alternate text while primary text is hidden</template>
+    <ng-template #elseBlock>Alternate text while primary text is hidden</ng-template>
 `
 })
 class NgIfElse {
@@ -51,9 +51,9 @@ class NgIfElse {
     show = {{show}}
     <br>
     <div *ngIf="show; then thenBlock; else elseBlock">this is ignored</div>
-    <template #primaryBlock>Primary text to show</template>
-    <template #secondaryBlock>Secondary text to show</template>
-    <template #elseBlock>Alternate text while primary text is hidden</template>
+    <ng-template #primaryBlock>Primary text to show</ng-template>
+    <ng-template #secondaryBlock>Secondary text to show</ng-template>
+    <ng-template #elseBlock>Alternate text while primary text is hidden</ng-template>
 `
 })
 class NgIfThenElse implements OnInit {
@@ -82,7 +82,7 @@ class NgIfThenElse implements OnInit {
     <div *ngIf="userObservable | async; else loading; let user">
       Hello {{user.last}}, {{user.first}}!
     </div>
-    <template #loading let-user>Waiting... (user is {{user|json}})</template>
+    <ng-template #loading let-user>Waiting... (user is {{user|json}})</ng-template>
 `
 })
 class NgIfLet {