Add key endorsements to process "signed statements" download

When using the https://nm.debian.org/process/<xxx>/download_statements/ link on an ongoing process, it would be nice if the key endorsements (visible in https://nm.debian.org/process/<xxx>/keycheck/), if any have been done, were added to the generated mbox.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information