Add email translations

Convert emails from Pug template to Handlebars because i18next doesn't
support Pug

server/core/initializers/migrations/0910-user-language.ts

@@ -0,0 +1,26 @@
+import * as Sequelize from 'sequelize'
+
+async function up (utils: {
+  transaction: Sequelize.Transaction
+  queryInterface: Sequelize.QueryInterface
+  sequelize: Sequelize.Sequelize
+}): Promise<void> {
+  const { transaction } = utils
+
+  {
+    await utils.queryInterface.addColumn('user', 'language', {
+      type: Sequelize.STRING,
+      defaultValue: null,
+      allowNull: true
+    }, { transaction })
+  }
+}
+
+function down (options) {
+  throw new Error('Not implemented.')
+}
+
+export {
+  down,
+  up
+}