Add a .gitignore_global file.

Not sure how much this is needed, but we saw fit to install it on the
old jenkins machine, so I'm making it part of the official config.

Auditors: chris

Test Plan:
None
Copy the old kiln Website -> tools -> naked-domain-redirect to domain-redirect.

This is the full contents of the domain-redirect ec2 machine.  (The
only contentful file is the .htaccess file.)

Auditors: alpert

Test Plan:
None