Restore support for animated GIFs in Gutenberg.

[SergioEstevao]

When we implemented our custom ImageLoader for RN, to support private sites, we accidentally removed support for playing animated GIFs while inside GB editor.
This PR changes the loading system of images to use the RCTAnimated class when a GIF is being downloaded.

One of the side effects of that change is that if the post featured image on the post/page is an animated GIF we show the animation.

Fixes #

To test:

• Start the app
• Create a new Post using GB
• Add an image block and configure it to use an animated GIF
• See if you can see the GIF animation on the editor
• Publish the post
• Go to the list of posts
• See if the post card in the list of posts shows the animated GIF
• Test this with WP.com sites, WP.org sites, Atomic sites, and WP.com private sites.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger an installable build for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[maxme]

The tests I ran worked fine, GIFs are animated on the post list, in Gutenberg (doesn't work on Aztec, but I guess that's expected). I'd appreciate if someone else was reviewing the code ;).

[etoledom]

Thank you @SergioEstevao for this fix!
I was wondering from some time already what happened to the animated gifs on iOS 😬

Added a couple of comments on the code, but everything works great!