Show site-url placeholder message when using MB_SITE_URL env var (#21731)

* Add propTypes to InputWithSelectPrefix

* Add placeholder prop

* Use setting.placeholder in SiteUrlWidget

frontend/src/metabase/admin/settings/components/widgets/SiteUrlWidget.jsx

-/* eslint-disable react/prop-types */
 import React, { Component } from "react";
+import PropTypes from "prop-types";
 
 import InputWithSelectPrefix from "metabase/components/InputWithSelectPrefix";
 
+const propTypes = {
+  setting: PropTypes.object.isRequired,
+  onChange: PropTypes.func.isRequired,
+};
+
 export default class SiteUrlWidget extends Component {
   render() {
     const { setting, onChange } = this.props;
@@ -13,7 +18,10 @@ export default class SiteUrlWidget extends Component {
         prefixes={["https://", "http://"]}
         defaultPrefix="http://"
         caseInsensitivePrefix={true}
+        placeholder={setting.placeholder}
       />
     );
   }
 }
+
+SiteUrlWidget.propTypes = propTypes;

frontend/src/metabase/components/InputWithSelectPrefix.jsx

-/* eslint-disable react/prop-types */
 import React, { Component } from "react";
+import PropTypes from "prop-types";
 
 import Select, { Option } from "metabase/core/components/Select";
 import InputBlurChange from "./InputBlurChange";
@@ -23,6 +23,15 @@ function splitValue({
   return prefix ? [prefix, value.slice(prefix.length)] : [defaultPrefix, value];
 }
 
+const propTypes = {
+  value: PropTypes.string,
+  prefixes: PropTypes.arrayOf(PropTypes.string),
+  defaultPrefix: PropTypes.string,
+  caseInsensitivePrefix: PropTypes.bool,
+  onChange: PropTypes.func,
+  placeholder: PropTypes.string,
+};
+
 export default class InputWithSelectPrefix extends Component {
   constructor(props) {
     super(props);
@@ -72,10 +81,12 @@ export default class InputWithSelectPrefix extends Component {
           type="text"
           className="Form-input flex-full borderless"
           value={rest}
-          placeholder={"foo"}
+          placeholder={this.props.placeholder}
           onBlurChange={e => this.setState({ rest: e.target.value })}
         />
       </div>
     );
   }
 }
+
+InputWithSelectPrefix.propTypes = propTypes;