Renamed email-preview file to email-previews

refs https://github.com/TryGhost/Toolbox/issues/308

- I recently pluralised the API endpoint but never made the changes to
  the controller file and everywhere else it's needed
- this commit cleans up that inconsistency so it should be clearer

core/server/api/canary/index.js

@@ -149,8 +149,8 @@ module.exports = {
         return shared.pipeline(require('./actions'), localUtils);
     },
 
-    get email_preview() {
-        return shared.pipeline(require('./email-preview'), localUtils);
+    get email_previews() {
+        return shared.pipeline(require('./email-previews'), localUtils);
     },
 
     get emails() {

core/server/web/api/canary/admin/routes.js

@@ -288,8 +288,8 @@ module.exports = function apiRoutes() {
     router.get('/actions', mw.authAdminApi, http(api.actions.browse));
 
     // ## Email Preview
-    router.get('/email_previews/posts/:id', mw.authAdminApi, http(api.email_preview.read));
-    router.post('/email_previews/posts/:id', mw.authAdminApi, http(api.email_preview.sendTestEmail));
+    router.get('/email_previews/posts/:id', mw.authAdminApi, http(api.email_previews.read));
+    router.post('/email_previews/posts/:id', mw.authAdminApi, http(api.email_previews.sendTestEmail));
 
     // ## Emails
     router.get('/emails', mw.authAdminApi, http(api.emails.browse));

test/e2e-api/admin/email-previews.test.js

@@ -54,7 +54,7 @@ describe('Email Preview API', function () {
             should.exist(jsonResponse);
             should.exist(jsonResponse.email_previews);
 
-            localUtils.API.checkResponse(jsonResponse.email_previews[0], 'email_preview', null, null);
+            localUtils.API.checkResponse(jsonResponse.email_previews[0], 'email_previews', null, null);
         });
 
         it('can read post email preview with email card and replacements', async function () {

test/e2e-api/admin/utils.js

@@ -165,7 +165,7 @@ const expectedProperties = {
     ,
     email: _(schema.emails)
         .keys(),
-    email_preview: ['html', 'subject', 'plaintext'],
+    email_previews: ['html', 'subject', 'plaintext'],
     email_recipient: _(schema.email_recipients)
         .keys()
         .filter(key => key.indexOf('@@') === -1),

test/regression/api/admin/utils.js

@@ -185,7 +185,7 @@ const expectedProperties = {
         'created_at',
         'updated_at'
     ],
-    email_preview: ['html', 'subject', 'plaintext']
+    email_previews: ['html', 'subject', 'plaintext']
 };
 
 module.exports = {