Allow LMS instructors to shuffle group membership between assignments so that students work with different groups over different assignments

[mkdir-washington-edu]

The problem
Instructors don't always create multiple LMS groups for a course, whether that's because the LMS makes it difficult, the instructor is unaware that they can, or some other reason. However, those instructors who want students using Hypothesis in small reading groups sometimes want to shuffle those groups so students aren't always working with the same group of students all semester.

The solution
Allow additional group creation within Hypothesis that isn't defined by an LMS group structure, adding features to make it easy for instructors to shuffle these groups between assignments.