Manage all seat counts using cohorts

[brendanheywood]

• [ ] split the admin UI into 2 forms, one for clients and one for a subscription
• [ ] each client subscription will be 1 client and 1 tier, but the tier could be changed. This means the 'subscription' has a unique internal id independent of the tier
• [ ] when you create or update a subscription, then auto create a cohort to match it which is tightly linked to this subscription id
• [ ] update its description so its obvious it is a 'managed' cohort BUT don't make it a real managed cohort as this means you can't manually edit the membership
• [ ] in the desc of the cohort have a link to the client subscription management ui
• [ ] remove all the existing logic which looks at enrollments in all the courses and unions them, this is complicated and overkill
• [ ] When auto enrolling check cohort size for seat counts first
• [ ] When enrolling add the person to the cohort
• [ ] The numbers inside a specific course should not matter
• [ ] If the cohort is emptied the users will still be in the courses and this is a way of resetting the seats each year