fix: change getAsset to not return a promise (#3232)

* fix: change getAsset to not return a promise

* fix: update markdown widget per getAsset changes

* test: fix editor component image test

* docs: update getAsset docs

setupTestFramework.js

@@ -5,4 +5,5 @@ import * as emotion from 'emotion';
 import { createSerializer } from 'jest-emotion';
 
 window.fetch = fetch;
+window.URL.createObjectURL = jest.fn();
 expect.addSnapshotSerializer(createSerializer(emotion));