Simpler cohort assignment

Runbooks

Cohort assignment management: assigning someone to a cohort is now a per-person action right on the assignees table, replacing a separate bulk-assign button that was confusing to use. You can also directly assign someone to a cohort from the parent runbook view.

Digest improvements: as well as calling out who hasn't been added to an onboarding runbook, the digest now links to the user's profile, so SuperAdmins can quickly identify whether this is expected or not.