Refactored image picker to js component

Also adjusted default cover image size

resources/assets/js/components/index.js

@@ -14,6 +14,7 @@ let componentMapping = {
     'wysiwyg-editor': require('./wysiwyg-editor'),
     'markdown-editor': require('./markdown-editor'),
     'editor-toolbox': require('./editor-toolbox'),
+    'image-picker': require('./image-picker'),
 };
 
 window.components = {};