Lint the webroot/rsrc/externals/javelin directory
Summary: Fixes T6953. As discussed, this directory should be linted.
Test Plan: Ran arc lint on a file in webroot/rsrc/externals/javelin and saw linter errors.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6953
Differential Revision: https://secure.phabricator.com/D11375