💄Updated fixtures for Ghost v3 content + assets (#11247)

no issue 

- Updated post images and content
- Updated default blog cover asset
- Updated default Ghost user avatar

core/server/data/schema/default-settings.json

@@ -37,7 +37,7 @@
             "defaultValue": "https://static.ghost.org/v1.0.0/images/ghost-logo.svg"
         },
         "cover_image": {
-            "defaultValue": "https://static.ghost.org/v1.0.0/images/blog-cover.jpg"
+            "defaultValue": "https://static.ghost.org/v3.0.0/images/publication-cover.png"
         },
         "icon": {
             "defaultValue": ""

core/test/unit/data/schema/integrity_spec.js

@@ -20,7 +20,7 @@ var should = require('should'),
 describe('DB version integrity', function () {
     // Only these variables should need updating
     const currentSchemaHash = '09ab28b8a18dc7f7f4fb5dbd5ccc10ac';
-    const currentFixturesHash = 'a15f00c333f207418952a9559fc3239f';
+    const currentFixturesHash = '59714063fa30b2f3fbfc929551b3c1e5';
 
     // If this test is failing, then it is likely a change has been made that requires a DB version bump,
     // and the values above will need updating as confirmation