Move Javelin to webroot

Summary:
This is a major pain on Windows and the main reason why Phabricator doesn't work there and is hard to fix.
The sad part is that Windows support symlinks (via `MKLINK`) but Git on Windows doesn't use them.

Test Plan: Loaded Phabricator on Windows without JS errors.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5458