[Improvement] Transition Character to Collection in back-end

Description

Transitioning Characters to a Collection will impact the structure of the stored data, so it'll be best to implement this before the First Playable release: otherwise, we'll effectively have to wipe the players' campaign data and start again.

Current State

Characters are handled separately from other collections on the back end. There's no real benefit to keeping these separate, it's just an artifact from an earlier iteration of the back end.

How should it look/feel?

Characters should be migrated to a Collection. Back-end methods dealing with Characters specifically should be removed. Front-end methods dealing with Characters should interact with the Collection instead.

Implementation

• On map tokens - campaign/collection/tokens
• In Library characters - user/collection/characters