title: Unable to add custom site config to Silverstripe 4
body: <p>Im following the <a href="https://docs.silverstripe.org/en/4/developer_guides/configuration/siteconfig/" rel="nofollow noreferrer">original docs</a> but <code>/dev/build</code> is not generating the db fields and the form fields are not getting generated either.</p>
  
  <p>this is the value of my <code>app/_config/app.yml</code></p>
  
  <pre><code>---
  Name: myproject
  ---
  SilverStripe\Core\Manifest\ModuleManifest:
    project: app
  ---
  Silverstripe\SiteConfig\SiteConfig:
    extensions:
      - CustomSiteConfig
  ---
  SilverStripe\Admin\LeftAndMain:
    extra_requirements_css:
      - public/resources/admin/css/custom.css
  </code></pre>
  
  <p>and this is my <code>app/src/extensions/CustomSiteConfig.php</code></p>
  
  <pre><code>&lt;?php
  
  use SilverStripe\Forms\FieldList;
  use SilverStripe\Forms\HTMLEditor\HTMLEditorField;
  use SilverStripe\ORM\DataExtension;
  
  class CustomSiteConfig extends DataExtension
  {
  
      private static $db = [
          'FooterContent' =&gt; 'HTMLText'
      ];
  
      public function updateCMSFields(FieldList $fields)
      {
          $fields-&gt;addFieldToTab("Root.Main",
              new HTMLEditorField("FooterContent", "Footer Content")
          );
      }
  }
  </code></pre>
  
  <p>Silverstripe is new to me and maybe I'm missing something here. But I'm looking for an hour now and cant get it to work.</p>