Replace spacer.gif by a base64 encoded string

Spacer.gif is used as a placeholder for images.

Currently, it is loaded from the web resources.

Since it's only a single empty pixel, it could be defined as a base64 string, which would save some client-server communication, and would not create longer URLs.

This would also prevent CORS issues with the embedding library trying to load spacer.gif from the original URL while embedded.

Current gif could be replaced with:

"data:image/gif;base64,R0lGODlhAQABAIAAAP///////yH5BAUUAAEALAAAAAABAAEAAAICTAEAOw=="