Implement a success metric for the instructor email digests feature

The [project document](https://www.notion.so/Increase-social-engagement-within-assignments-9b32fe7f7cc84d969e209d58b024dbaf) says that the outcome should be **an increase in the amount of times a user returns after their first session.**

**Question:** returns to what? Are we talking about the same user returning multiple times to the same assignment? Or are we just talking about a user launching Hypothesis again (even if it's in a different assignment or course)? **I think the answer is that we want an increase in how often users re-launch the same assignment**. How can we measure this "re-launch rate" so that we can tell whether it increases after we ship this feature?

**Answer:**

We'll create a metric that measures how many times users re-launch the same assignment.