Add handling of differing layout versions

[kittsville]

Chomskey will need to be able to perform sanitisation checks relevant to the version of Chomskey an uploaded layout was intended for. Chomskey must also be able to (if necessary) upgrade that layout to the current version of Chomskey.

Given only two people currently know about Chomskey this issue can be held off by simply rejecting all layouts not designed for the current version.

[kittsville]

Next step is to add the ability to migrate layouts from older versions of Chomskey. Considering nobody currently uses Chomskey I'd rank this as low priority over solving issues that would make more people want to use Chomskey in the first place.