Locked attestations do not properly expire

The creation_date is correctly set for all locked attestations. The publication_date is set for all public attestations. The only thing we need to implement to get this going is:

• A warning on the locked attestation page that tells the user about the remaining unlock time (which is a constant on the settings.py file for the project.
• A cronjob that runs a (very simple) script that, for all attestations w/o publication_date, compares the current date to the creation date and deletes that attestation (just the attestation, not the experiment) if the time difference pass a threshold.