Add UI controls to the workspace for custom cohorts

[mitalia]

Users need to have a place in the UI of the workspace to see the available cohorts. This basic UI can probably use standard Harvest object set functionality. Ideally, the UI should list the cohorts based on their level: personal, team, or global. I will detail cohort types in another ticket. For cohorts that users can manage, they should be able to create new ones, edit existing ones (which will increment the version), and delete cohorts. Exact behavior will be in another ticket.