Class management features

Since CodeWorld is intended for educational use, there are a number of features to help in running a class of students learning together, which would be worth considering. I'm opening this issue to brainstorm some of them.

* Submission boxes can be created that collect submitted projects.
* A way to attach comments to student projects.
* Galleries can be submitted for students to share projects with their classmates.
* Teachers can post reference/example programs. This is especially useful for activities where students are asked to start with some base program and modify it.

Another alternative would be integrating CodeWorld with some other system, like Google Classroom, that already provides many of these features.

No files, tests, or entry points are named; the issue is an open brainstorming list rather than a defined implementation task. Start by determining whether CodeWorld should provide these class-management features or integrate with another system, then establish one scoped feature and its acceptance criteria before locating the relevant code.

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