Add cron to purge completely blank proposals

[dternyak]

Blank proposals are generated and stored in the database immediately when hitting the proposal creation view.

These can clog up the user drafts view, as well as the admin proposals view.

We can add a check to see if a proposal has been created and stayed completely blank for more than 24 hours, after which we can delete it.

[wbobeirne]

Closing to roll this in with #198.