Bugfix: Add WAGTAILADMIN_BASE_URL in the get_media_item_url function

[Hercilio1]

The current Wagtail version removed BASE_URL, so to ensure the get_media_item_url function works, I updated it to try both, prioritizing WAGTAILADMIN_BASE_URL.

I'm having issues with my current project when trying to get the image URL using the following GraphQL query:

{
  pages {
    ... on PageForTests {
      id
      title
      body {
        ... on CustomImageBlock {
          id
          altText
          image {
            url
          }
        }
      }
    }
  }
}