Replaced i18n.t w/ tpl in v2/slugs.js (#13505)

refs: TryGhost#13380

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

core/server/api/v2/slugs.js

@@ -1,7 +1,11 @@
 const models = require('../../models');
-const i18n = require('../../../shared/i18n');
+const tpl = require('@tryghost/tpl');
 const errors = require('@tryghost/errors');
 
+const messages = {
+    couldNotGenerateSlug: 'Could not generate slug.'
+};
+
 const allowedTypes = {
     post: models.Post,
     tag: models.Tag,
@@ -37,7 +41,7 @@ module.exports = {
                 .then((slug) => {
                     if (!slug) {
                         return Promise.reject(new errors.GhostError({
-                            message: i18n.t('errors.api.slugs.couldNotGenerateSlug')
+                            message: tpl(messages.couldNotGenerateSlug)
                         }));
                     }
                     return slug;