Different type of Key needed: not claimable until specific date

[kaleissin]

The current design is good for keys generated on-the-fly but not so good for keys valid during an event with a defined start and end, like tokens at a festival.

Add attribute 'created' on the Key, copied from pub_date for existing keys, then as an 'auto_add'. Add attributes 'valid_from' (may be None) and 'expires' (may be None) on KeyGroup. This sets 'pub_date' and 'expires' on the Key. Ensure that if 'valid_from' is set, the Key cannot be claimed before that date. Issue a warning/error if both 'expires' and 'ttl' set on the KeyGroup. No 'pub_date' on Key: always valid? Copy from Key.created?

Add a filter to admin to show valid vs. invalid keys.

Make conversion-script (sql? migrations? docs only?) and bump major version.