Added type:post enforcement when fetching posts for collections

refs https://github.com/TryGhost/Team/issues/3148

- Collections should never ever operate on pages, so limiting them at the fetch stage is the bes possible place, so they don't enter the system when an automatic collection is created.

ghost/core/core/server/services/collections/index.js

@@ -15,7 +15,8 @@ class CollectionsServiceWrapper {
             postsRepository: {
                 getAll: async ({filter}) => {
                     return models.Post.findAll({
-                        filter
+                        // @NOTE: enforce "post" type to avoid ever fetching pages
+                        filter: `(${filter})+type:post`
                     });
                 }
             }