Add badge for signing up on CSOL.

[marihuertas]

This will be a participation badge for joining CSOL and committing to learning throughout the summer – we will email all those who already are on the site and say they’ve earned it. This makes it a plus for those who haven’t joined CSOL yet.

I'm asking MKCPR, the PR firm MacArthur hired, to design.

Slating here just so I can keep tabs on it.

[threeqube]

@iamjessklein created the design for this. Issue here: #180 (comment)