Pull htmlbars in via bower.

This allows much simpler updating to new versions, just update the sha in `bower.json`, run `bower update htmlbars` and boom: new morph version.  You can also, now very easily `bower link` in the htmlbars repo (for live updating during development).

Closes #5227.

**Please Note:** This targets the `metal-views` branch.

/cc @mixonic, @mmun, @krisselden