Add static square image for OAuth

OAuth providers typically ask for an asset that they can use
to co-brand when you authenticate.  We needed a square
120x120px image for Google, of which we're using the shield.

Put this in `/public/static` instead of assets because
we want a known file name that isn't fingerprinted.
Merge pull request #348 from 18F/hotfix/image-for-google-oauth

Add static square image for OAuth