Rename standard-app-packages to meteor-platform
Add back gitignore
Rename standard-app-packages to meteor-platform

Summary: Also update the skeleton app to use meteor-platform

Test Plan: Created a new app, it had meteor-platform, it ran

Reviewers: dgreenspan

Reviewed By: dgreenspan

CC: avital, emily, nim

Differential Revision: https://phabricator.meteor.com/D785