Blank logo 512x512

Summary:

• export SVG to 480x451 PNG
• extend with ImageMagick to a 512x512

ImageMagick command:

$ convert logo-blank.svg \
          -gravity center \
          -background transparent \
          -extent 512x512 \
          logo-blank-512px.png

Fixes T568.

Test Plan: test on Docker Hub

Reviewers: dereckson

Projects: design

Maniphest Tasks: T568

Differential Revision: http://devcentral.nasqueron.org/D60