add placeholder to Select components (#3274)

superset/assets/javascripts/explore/components/SaveModal.jsx

@@ -188,6 +188,7 @@ class SaveModal extends React.Component {
             onChange={this.onChange.bind(this, 'saveToDashboardId')}
             autoSize={false}
             value={this.state.saveToDashboardId}
+            placeholder="Select Dashboard"
           />
 
           <Radio

superset/assets/visualizations/filter_box.jsx

@@ -66,6 +66,7 @@ class FilterBox extends React.Component {
           <div className="m-b-5" key={field}>
             {field.replace('__', '')}
             <Select.Creatable
+              placeholder="Select"
               options={options}
               value={this.state.selectedValues[field]}
               onChange={this.changeFilter.bind(this, field)}