Relaxed validation rules + removed unknown properties

refs https://github.com/TryGhost/Toolbox/issues/314

- The API principle guiding this change is the Robustness Principle: "be conservative in what you send, be liberal in what you accept". The API will start accepting any additional properties that are not explicitly defined in the schema for the resource and will be trimming any rogue properties that are sent in the payload

Author: naz

packages/admin-api-schema/lib/utils/json-schema.js

@@ -7,6 +7,7 @@ const errors = require('@tryghost/errors');
 const ajv = new Ajv({
     allErrors: true,
     useDefaults: true,
+    removeAdditional: true,
     formats: {
         'json-string': (data) => {
             try {

packages/admin-api-schema/test/api.test.js

@@ -66,6 +66,26 @@ describe('Exposes a correct API', function () {
             }
         });
 
+        it('Unknown fields get ignored and trimmed', async function () {
+            const data = {
+                posts: [{
+                    title: 'valid',
+                    author: 'Beccy',
+                    something: 'else'
+                }]
+            };
+
+            try {
+                should.equal(data.posts[0].something, 'else');
+                should.equal(data.posts[0].author, 'Beccy');
+                await apiSchema.validate({data, schema: 'posts-add', definition: 'posts'});
+                should.equal(data.posts[0].something, undefined);
+                should.equal(data.posts[0].author, undefined);
+            } catch (err) {
+                throw new Error('should not throw an error');
+            }
+        });
+
         it('Incorrect use throws an error', async function () {
             const data = {
                 posts: [{