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

packages/netlify-cms-core/src/routing/history.js

@@ -1,5 +1,5 @@
 import { createHashHistory } from 'history';
 
-let history = createHashHistory();
+const history = createHashHistory();
 
 export default history;