Mail: Add option to auto-show insecure parts

In some situations it would be helpful to have the "Show parts not being secured" action for partially GPG signed/encrypted mails done by default. This could be done through either another preference, or just a gsettings key that people can change. The preferences dialogs are rather full though and probably this will be a rather niche feature.

On the debian-newmaint mailing list, almost mails are posted through a web service, with an inline signed part submitted to the website by a person, and a non-signed part provided by the service that links to the ticket for the submission, which means you have to click twice to get the link.