Add support for background images within Elementor

[gabrielcld2]

This ensures that background images generated within Elementor's CSS files are handled accordingly.

Approach

• Hook into the Elementor parse_css hook.
  • Check for any background image elementor keys
  • Generate the corresponding cloudinary URL for the media
  • Inject the right CSS rule to override the default one within Elementor
• Flush the elementor cache whenever a cache flush occurs on Cloudinary
  • This ensures any setting change is directly applied for Elementor background images

QA notes

• Activate the Elementor plugin
  • Create a page with Elementor
    • Within the elementor page, add an element like a heading
    • Edit the heading, go to Advanced and open the background section
    • Apply a background image (and if you want, you can also apply a hover background image, or even some viewport specific ones like tablet or mobile background images)
  • Publish and view the page
  • Inspect the element and check its CSS contains the cloudinary URL for the background image
  • Go to Cloudinary -> Image Settings
    • Make some changes for instance the image format
    • Save the changes
    • Reload the elementor page and make sure the changes applied to the background image
• Deactivate the Elementor plugin
  • Make sure the Cloudinary plugin still works, and doesn't expect Elementor to be enabled