VRFS-2916 cohort model/table

admin/app/models/cohort.rb

@@ -0,0 +1,4 @@
+class Cohort < ActiveRecord::Base
+
+end
+

db/up/cohorts.sql

@@ -0,0 +1,9 @@
+CREATE TABLE cohorts {
+    id VARCHAR(64) PRIMARY KEY DEFAULT uuid_generate_v4(),
+
+    start_date TIMESTAMP NOT NULL,
+    end_date TIMESTAMP NOT NULL,
+    group_type VARCHAR(64) NOT NULL,
+    
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+};