Replaced i18n.t w/ tpl in v2/preview.js (#13503)

refs: TryGhost#13380

- The i18n package is deprecated. It is being replaced with the tpl package.

core/server/api/v2/preview.js

@@ -1,8 +1,12 @@
-const i18n = require('../../../shared/i18n');
+const tpl = require('@tryghost/tpl');
 const errors = require('@tryghost/errors');
 const models = require('../../models');
 const ALLOWED_INCLUDES = ['authors', 'tags'];
 
+const messages = {
+    postNotFound: 'Post not found.'
+};
+
 module.exports = {
     docName: 'preview',
 
@@ -31,7 +35,7 @@ module.exports = {
                 .then((model) => {
                     if (!model) {
                         throw new errors.NotFoundError({
-                            message: i18n.t('errors.api.posts.postNotFound')
+                            message: tpl(messages.postNotFound)
                         });
                     }