Move get_cohort_by_name() to CohortService

Move wikimetrics.controllers.cohorts.get_cohort_by_name() to
wikimetrics.api.cohorts.CohortService.

Change-Id: I3c0b89bf3a702e43bacc9884d2baaf608cea1e0b
Add validators module and use it in cohort_upload

Change-Id: Ic1c2aeb38d812ab9bdf016d87bc6090d8f3eba78