Add LMS instance names and links to instructor email digests

Evolve the email template to look like this:

> **Subject:** There has been some activity in Hypothesis
>
> **Body:**
>
> Hi,
>
> Your students made **64 annotations** in assignments at **Hypothesis University** yesterday. Let your students know about the activity to encourage the ongoing conversation.
>
> Hypothesis Team

See https://github.com/hypothesis/lms/issues/4915#issuecomment-1405352481