Fixed Tier events being created when Posts are edited

refs https://github.com/TryGhost/Team/issues/1875

- due to an misbehavior in our model layer, when `tiers` is set on a Post, it'll
  trigger a save of the Tier, and this produces an extra event in the
  `actions` table
- mapping the Tier(s) to just the ID prevents bookshelf-relations from
  editing the Tier and thus prevents the extra event
- also fixed tests which were implicitly assuming supplying a slug to a
  post would create the product

ghost/core/core/server/models/post.js

@@ -794,6 +794,12 @@ Post = ghostBookshelf.Model.extend({
             });
         }
 
+        if (this.get('tiers')) {
+            this.set('tiers', this.get('tiers').map(t => ({
+                id: t.id
+            })));
+        }
+
         return sequence(ops);
     },
 

ghost/core/test/e2e-frontend/helpers/get.test.js

@@ -72,11 +72,13 @@ describe('e2e {{#get}} helper', function () {
             published_at: new Date() // here to ensure sorting is not modified
         });
 
+        const defaultTier = await models.Product.findOne({slug: 'default-product'});
+
         basicTierPost = await createPost({
             slug: 'tiers-post',
             visibility: 'tiers',
             tiers: [{
-                slug: 'default-product'
+                id: defaultTier.get('id')
             }],
             published_at: new Date() // here to ensure sorting is not modified
         });

ghost/core/test/e2e-frontend/helpers/next_post.test.js

@@ -54,11 +54,13 @@ describe('e2e {{#next_post}} helper', function () {
             published_at: new Date(2020, 0, 3) // here to ensure sorting is not modified
         });
 
+        const defaultTier = await models.Product.findOne({slug: 'default-product'});
+
         basicTierPost = await createPost({
             slug: 'tiers-post',
             visibility: 'tiers',
             tiers: [{
-                slug: 'default-product'
+                id: defaultTier.get('id')
             }],
             published_at: new Date(2020, 0, 4) // here to ensure sorting is not modified
         });