Replaced i18n.t w/ tpl helper in memberSigninUrls.js (#13462)

refs: #13380

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

core/server/api/canary/memberSigninUrls.js

@@ -1,7 +1,11 @@
-const i18n = require('../../../shared/i18n');
+const tpl = require('@tryghost/tpl');
 const errors = require('@tryghost/errors');
 const membersService = require('../../services/members');
 
+const messages = {
+    memberNotFound: 'Member not found.'
+};
+
 module.exports = {
     docName: 'member_signin_urls',
     permissions: true,
@@ -15,7 +19,7 @@ module.exports = {
 
             if (!model) {
                 throw new errors.NotFoundError({
-                    message: i18n.t('errors.api.members.memberNotFound')
+                    message: tpl(messages.memberNotFound)
                 });
             }