Moved CollectionPost type to it's own file

refs https://github.com/TryGhost/Arch/issues/41

- This type has to be reused in other class, so best way to do it is having it separated into it's own file

ghost/collections/src/Collection.ts

@@ -2,6 +2,7 @@ import {UniqueChecker} from './UniqueChecker';
 import {ValidationError} from '@tryghost/errors';
 import tpl from '@tryghost/tpl';
 import nql = require('@tryghost/nql');
+import {CollectionPost} from './CollectionPost';
 
 import ObjectID from 'bson-objectid';
 
@@ -16,12 +17,6 @@ const messages = {
     slugMustBeUnique: 'Slug must be unique'
 };
 
-type CollectionPost = {
-    id: string;
-    featured?: boolean;
-    published_at?: Date;
-}
-
 export class Collection {
     id: string;
     title: string;

ghost/collections/src/CollectionPost.ts

@@ -0,0 +1,7 @@
+// eslint-disable-next-line ghost/filenames/match-regex
+export type CollectionPost = {
+    id: string;
+    featured?: boolean;
+    published_at?: Date;
+    tags: string[];
+};