Method for updating crontab

After #45 (closed), the tag2upload-oracle user's crontab is going to become nontrivial. We want to be able to update it.

I suggest: dgit-infrastructure has a postinst script which does crontab -u iff the tag2upload-oracle user exists and already has a crontab.

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