Deprecate ng-template[polymer] feature

[hotforfeature]

Related to angular/angular#17051

Once enableLegacyTemplate is correctly working, PolymerTemplateDirective should first check and see if this compiler option is set to false. If it is not, a deprecation warning should be generated.

PolymerTemplateDirective ends up doing quite a bit more than the original concept of detaching and reattaching a template. The directive won't be removed entirely. More than likely, it should change to select ng-template[polymer], template and automatically apply itself to templates.

Until angular/angular#15557 is fixed, this directive should also attempt to move any children of the template to the template's content property.