Don't load platform.js

Crosswalk supports all the features that Polymer wants, so we don't
need to shim them.
Disable selection for item list
Prevent undoing dismissal from causing a link to be clicked
Implement item dismissal
Improve Social app title
feed: make it possible to load more items
fix extra data generation
Load more items on-demand ("infinite scroll")

We actually stop at 1000 so that we'll use finite memory.