Avatars

We want a feature of the web site where a student can choose to capture a screen from a running CodeWorld program, and set it as their avatar. The avatar has the following uses:

1. It is displayed in the UI when logged in, making it easy to see whether you are logged in, or someone else is logged in on a shared computer.
2. Eventually, it can be used for collaboration features, such as collaborative code editing, comments, and submitting code to a gallery. (e.g., the kinds of features in #551)
3. We want it for motivation reasons. The official curriculum will start with making student name tags, and then students may want to customize from there.