fix(db): add missing subtype index to river table

Fixes #10896

engine/schema/migrations/20170802080931_add_subtype_index_to_river_table.php

@@ -0,0 +1,38 @@
+<?php
+
+use Phinx\Migration\AbstractMigration;
+
+class AddSubtypeIndexToRiverTable extends AbstractMigration {
+	/**
+	 * Change Method.
+	 *
+	 * Write your reversible migrations using this method.
+	 *
+	 * More information on writing migrations is available here:
+	 * http://docs.phinx.org/en/latest/migrations.html#the-abstractmigration-class
+	 *
+	 * The following commands can be used in this method and Phinx will
+	 * automatically reverse them when rolling back:
+	 *
+	 *    createTable
+	 *    renameTable
+	 *    addColumn
+	 *    renameColumn
+	 *    addIndex
+	 *    addForeignKey
+	 *
+	 * Remember to call "create()" or "update()" and NOT "save()" when working
+	 * with the Table class.
+	 */
+	public function change() {
+
+		$table = $this->table('river');
+
+		$table->addIndex(['subtype'], [
+			'name' => "subtype",
+			'unique' => false,
+		]);
+
+		$table->save();
+	}
+}