Move all external JS into webroot/rsrc/externals

Summary:
Currently, Celerity map rebuilds on Windows don't put Stripe or Raphael into the map. Move them into `webroot/rsrc/externals/` so they get picked up.

At some point we should maybe let the mapper load resources from mulitple locations, but this is more straightforward for now.

See https://github.com/facebook/phabricator/issues/294

Test Plan: Rebuilt map, verified Burnup Rate + Stripe work.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5661

Author: epriestley