Hygiene: Simplify {{#eachPost}} handlebars helper

* Move the iteration inside eachPost, it previously wasn't due to a limitation
  of lightncandy helpers, but the new-ish hbhelpers can handle iteration directly.
* change failure case to render a placeholder instead of throwing an exception
* make php and js implementations match eachother

Change-Id: I15437958e944d96562c58b0bf3fac7e78ec2188a