allow opening messages of unlimited size + add download progress

[tomholub]

Currently, messages with attachments over 5mb are disabled.

I'd like to re-enable them but during loading, show progress spinner. And allow user to use the back icon in case they don't want to wait.

Sometime in the future, we'll separate the logic to only pull message body and not attachments first, but that's not trivial for OpenPGP messages (content is often in attachments) so that will later need a way to differentiate which attachments to pull. I'll file another issue for that.