impliment editing and deleting cohorts

`respond_to?` tests to see if the string/symbol passed is a valid method that can be called. There was a bug with the `current_cohort_path` when the user was on the edit page. On that page, the url is /cohorts/edit which gets translated into `"cohort_edit_path"`. I considered testing `if cohort_referer == 'edit'` to determine where to put the `cohort_referer`, but it felt too specific. The `respond_to?` solution isn't my favorite, but it handles all our cases and prevents us from getting no method errors.