Add migration to convert first task on all surveys to post stage

All surveys need a 'post' stage. We'll convert the first stage
to a post stage if it isn't done already

migrations/20160902084943_migrate_first_task_to_post_stage.php

@@ -0,0 +1,58 @@
+<?php
+
+use Phinx\Migration\AbstractMigration;
+
+class MigrateFirstTaskToPostStage extends AbstractMigration
+{
+    /**
+     * Migrate Up.
+     */
+    public function up()
+    {
+        $pdo = $this->getAdapter()->getConnection();
+
+        // Get first stage for each form
+        $rows = $this->fetchAll(
+            "SELECT a.id
+                FROM form_stages a
+                LEFT JOIN form_stages b             -- JOIN for priority
+                ON (
+                    b.form_id = a.form_id
+                    AND b.priority < a.priority
+                    )
+                LEFT JOIN form_stages c             -- JOIN for priority ties
+                ON (
+                    c.form_id = a.form_id
+                    AND c.priority = a.priority
+                    AND c.id < a.id
+                    )
+                WHERE b.id IS NULL AND c.id IS NULL AND a.form_id NOT IN (
+                    SELECT form_id from form_stages WHERE form_stages.type = 'post'
+                )"
+        );
+
+        $update = $pdo->prepare(
+            "UPDATE
+                form_stages fs
+            SET type = 'post', required = 0
+            WHERE id = :id"
+        );
+
+        foreach ($rows as $row) {
+            $update->execute(
+                [
+                    ':id' => $row['id']
+                ]
+            );
+        }
+    }
+
+    /**
+     * Migrate Down.
+     */
+    public function down()
+    {
+        $this->execute("UPDATE form_stages fs SET type = 'task', required = 1, priority = 0
+            WHERE type = 'post'");
+    }
+}