Cohort Management Cleanup

We're tightening up how cohorts behave across the subscriber list and the announcement editor. Cohort subscriber counts will include domain-based members, so a cohort defined by email domain no longer reads as empty when it's actually reaching hundreds of people. You'll be able to assign cohorts directly from the Add and Edit Subscriber dialogs, and the cohort "Add" box will pick from your existing subscribers instead of accepting addresses that can't receive anything. We're also making bulk actions respect an applied cohort filter, and clarifying in the editor that cohorts add to your category and roadmap audience rather than narrowing it.