cards format not y2k+100 compliant

[themusicgod1]

It's not possible to put a 4 year date in the cards date metadata. This means come 2099 or so, we're going to have some confusing card decks. (Incidentally, decks that "pre-date" the 2000s are also going to be mistagged).

To reproduce
1) create a deck of cards
2) file
3) export
4) enter a name in the "file to export to" field
5) OK
a new window comes up to edit metadata for this deck
including a 'date' field.
6)

what does happen:
only allows dates inside of range 01-99 (eg within 2001-2099)

what should happen:
format should allow dates outside of range 01-99 (eg outside 2001-2099)

(This can probably wait until a "Mnemosyne 3.x" cards format)

mnemosyne: 2.4-0.1
ubuntu: bionic 18.04