Challenge identifiers should be UUID's

[k9delight]

If challenge identifiers were UUID's, it would give us a variety of features, such as:

• the ability to incrementally add levels and merge/restore them without likelyhood of conflict with existing levels
• "level packs" where we can distribute the level descriptions afterwards, and people can add them to their own instance

[Matir]

I completely agree, but more so, I think we should do the same for:

• Categories (or possibly deterministic keys for merging)
• Hints

Basically anything that's attached to the Challenges.