Create Cohort Controller

Creates a CohortController class.

• Issue #76 (closed) asks for a method to get all cohorts for a program. This makes no sense, as a cohort isn't directly related to a program. A cohort isn't CSE-2019, or AP-2019, it's just 2019. We might need to discuss this some more, but it's out of scope for this issue.

Closes #76 (closed)