Avoid unnecessary network traffic and cpu/mem load when saving message drafts

Saving attachments may generate unneeded network and cpu because it modifies document contents. We need to evaluate if that is a problem.

For that, a first step is to get a clear picture of how email drafts work in both the pixelated and thunderbird use case.

This might need to consider drafts a special case of blobs, w/o breaking soledad generality.