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.

aio/content/cheatsheet/built-in-directives.md

@@ -20,9 +20,9 @@ Turns the li element and its contents into a template, and uses that to instanti
 @cheatsheetItem
 syntax:
 `<div [ngSwitch]="conditionExpression">
-  <template [ngSwitchCase]="case1Exp">...</template>
-  <template ngSwitchCase="case2LiteralString">...</template>
-  <template ngSwitchDefault>...</template>
+  <ng-template [ngSwitchCase]="case1Exp">...</ng-template>
+  <ng-template ngSwitchCase="case2LiteralString">...</ng-template>
+  <ng-template ngSwitchDefault>...</ng-template>
 </div>`|`[ngSwitch]`|`[ngSwitchCase]`|`ngSwitchCase`|`ngSwitchDefault`
 description:
 Conditionally swaps the contents of the div by selecting one of the embedded templates based on the current value of `conditionExpression`.

aio/content/cheatsheet/template-syntax.md

@@ -65,7 +65,7 @@ syntax:
 `<p *myUnless="myExpression">...</p>`|`*myUnless`
 description:
 The `*` symbol turns the current element into an embedded template. Equivalent to:
-`<template [myUnless]="myExpression"><p>...</p></template>`
+`<ng-template [myUnless]="myExpression"><p>...</p></ng-template>`
 
 @cheatsheetItem
 syntax: