Remove support for custom JS dependencies in project type spec

This form of project type specification in spark_polymer_ui.html is no longer used, because it used to put explicit knowledge about Polymer into spark.dart: "js-web-app-polymer;polymer,paper-elements".

Conversely, since the instantiation code already had some support for multi-template projects, fully implemented it (it was very easy and quick), so the "template-a+template-b+templace-c" form is now supported.
Merge pull request #3764 from ussuri/cleanup-templates-infra

Remove support for custom JS dependencies in project type spec