Don't allow replies with a broader set of permissions than the annotation they're replying to

[mkdir-washington-edu]

This is solution A from this document: #1602.

The Problem
Students can create shared (annotation.shared = TRUE) replies to unshared (annotation.shared = FALSE) annotations. These shared replies are counted in our email digest and by our Auto-grading feature. They're also not visible to anyone except the user who owns the parent unshared annotation.

The Solution
While there are other potential solutions that cover various edge cases of the main problem, we predict that the largest source of this phenomenon is students creating their own unshared annotations (such as highlights) and created shared replies to them. This will stop that practice, which is the correct first step.

At the moment a reply is being posted, check the shared status of the reply and of the parent annotation. Do not allow users to save shared replies if the parent annotation is unshared.