Allow to supply custom js imports in admin/index.html generated by gatsby-plugin-netlify-cms

[lrq3000]

NetlifyCMS often rolls out custom features and there begins to be an ecosystem of custom widgets. It would be nice to allow in the gatsby-plugin-netlify-cms plugin's config to customize the admin.html, or to be able to shadow its output when a /static/admin/index.html is present in the gatsby site's source.

One example use case is the latest netlifyCMS which includes the beta support for nested collections (ie, recursive discovery of nodes). To improve the user experience, there is a 3rd-party widget (meta parent widget type) which allows to select in a dropdown list the parent node instead of having to specify a path string manually (see here for more infos).

To include the parent widget I would need to add the following in the generated admin/index.html:

    <script src="https://unpkg.com/@netlify/netlify-cms-widget-parent@^1.0.0/dist/netlify-cms-widget-parent.js"></script>
    <script>
      const parentWidget = window.NetlifyCmsWidgetParent;
      CMS.registerWidget('parent', parentWidget.control, parentWidget.preview);
    </script>

Currently the only way I have found is to simply copy my own /static/admin/index.html after gatsby build by setting the copy command inside netlify.toml:

[build]
publish = "public"
command = "npm run build; yes | cp static/admin/index.html public/admin/index.html"

I expect this need to increase in the future with more and more widgets available for NetlifyCMS.