Bug 985454 - [Home2] App Grid Zooming and Reflow

https://bugzilla.mozilla.org/show_bug.cgi?id=985454

Note: Due to time sensitivity, homescreen2 work is currently landing un-reviewed for code within test_apps. A full review pass will happen before merging into apps/. If you have any drive-by comments/concerns please leave them and I will address them.