Removed unnecessary authors serializer

refs: https://github.com/TryGhost/Toolbox/issues/245

- we don't need this serializer because the default serializer will call the authors mapper
- added an author mapper which is just an alias for users. I did this in a named file, not in index.js
  - because we want to change the api framework to load files automatically without needing index files.
  - makes sense to hold off that deeper change until we only have one api version else we have to change old APIs

core/server/api/canary/utils/serializers/output/authors.js

@@ -1,21 +0,0 @@
-const debug = require('@tryghost/debug')('api:canary:utils:serializers:output:authors');
-const mappers = require('./mappers');
-
-module.exports = {
-    browse(models, apiConfig, frame) {
-        debug('browse');
-
-        frame.response = {
-            authors: models.data.map(model => mappers.users(model, frame)),
-            meta: models.meta
-        };
-    },
-
-    read(model, apiConfig, frame) {
-        debug('read');
-
-        frame.response = {
-            authors: [mappers.users(model, frame)]
-        };
-    }
-};

core/server/api/canary/utils/serializers/output/index.js

@@ -109,10 +109,6 @@ module.exports = {
         return require('./oembed');
     },
 
-    get authors() {
-        return require('./authors');
-    },
-
     get config() {
         return require('./config');
     },

core/server/api/canary/utils/serializers/output/mappers/authors.js

@@ -0,0 +1 @@
+module.exports = require('./users');

core/server/api/canary/utils/serializers/output/mappers/index.js

@@ -1,5 +1,6 @@
 module.exports = {
     actions: require('./actions'),
+    authors: require('./authors'),
     emails: require('./emails'),
     images: require('./images'),
     integrations: require('./integrations'),