Adhere to code conventions

Summary:
As this repository is a resource to help with code style,
files inside should be correctly formatted:

• use 4 spaces for XML
• apply them to a hird-party pre-commit hook

"We eat our own dog food."

Test Plan: ec (editorconfig-checker)

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3175