R1:294f2bf1184d

Don't lint `/webroot/rsrc/externals/`.

Summary: If I understand correctly, the `/webroot/rsrc/externals/` directory should be treated similarly to `/externals/` in terms of linting and coding style.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9689