Devoncarew stagehand

Use stagehand for some of our dart project templates:

- change the dart web template over to the one in stagehand
- add stagehand's dart package template
- remove the dart polymer app template - it generally doesn't run for users w/o them knowing that it has be run in Dartium

also
- created a new `StagehandProjectTemplate` class
- moved the existing templates to be subclassed by a new `SparkProjectTemplate` class
- made `ProjectTemplate` be an abstract, mostly empty class

@ussuri 
Not an action point: I've been thinking about this, and just filed my thoughts in #3617.