add AuthService as a DM authentication handler. (NEEDS SECURITY PASS)

models/AuthChallenge.js

@@ -0,0 +1,14 @@
+// @flow
+import type Sequelize, { DataTypes as DT } from 'sequelize'
+
+export default (sql: Sequelize, DataTypes: DT) => {
+  return sql.define('auth_challenge', {
+    userId: DataTypes.TEXT,
+    issuedAt: DataTypes.DATE,
+    type: DataTypes.ENUM('dm', 'other'),
+    human: { type: DataTypes.TEXT, unique: true },
+    magic: { type: DataTypes.TEXT, unique: true }
+  }, {
+    indexes: [ { fields: ['userId'] } ]
+  })
+}