Move frequency data to top level when exporting for Translatewiki
Summary: Ref T5267. Generate a single big frequency.json at the project root instead of separating it across projects.
Test Plan: Ran exporters, spot-checked generated frequency files (this data is still random).
Reviewed By: chad
Maniphest Tasks: T5267
Differential Revision: https://secure.phabricator.com/D16932