Updated github mark for notmyidea

The smallest mark GitHub provides in the bundle is 32x32. Since they
explicitly say don't resize the mark, I asked them to provide a 16x16
so we could use it. I received the 16x16 in this commit from GitHub
support.

However, they suggested we use the 32x32 and use CSS rules to adjust
the size:

"...we use the 32px for both and employ css background and background-
size rules to handle retina displays. I recommend that technique,
since retina users will see a mess of pixels for icons that small."

This sounds like reasonable advice. Should we consider migrating the
notmyidea social icons to 32x32, with appropriate CSS to adjust the
sizes?