GIF Block: Smoother image swapping in editor

When GIFs are swapped in the editor, there is a white flash that is jarring:


Smooth out this experience. Noted: https://github.com/Automattic/wp-calypso/pull/30402#discussion_r250910439

cc: @thomasguillot @jeffersonrabb

Ideas:
- Is it smoother if we don't use iframes and avoid loading different documents? Even with images that should be cached, there's always a flash
- Use multi-step transition with 2 iframes. This is much more complex than swapping an attibute on the iframe. Would look something like:
1. On search
1. Load 2nd iframe opacity 0 iframe over the current iframe
1. Use the new iframe's onload event to trigger a transition to opacity 1
1. Remove the old iframe when transition is complete