Add warnings about unsupported G Sync actions
* src/mail.cpp (Mail::update_oom_data): Save if account is gsync account. (Mail::encrypt_sign): Warn / Block actions for GSync accounts. * src/mimemaker.cpp (count_usable_attachments): Expose so that it can be used in Mail.
It's better to fail early and loudly instead of creating
broken mails.
- GnuPG-Bug-Id: T3545