feat(difficulty): Add difficulty settings to user schema

Lays the initial groundwork for the difficulty settings described at https://trello.com/c/C7CXSo02/428-difficulty-settings-v1. No functional changes at this time, just creating an object in the user schema for later manipulation as these features are implemented. @deilann
Merge branch 'difficulty' of https://github.com/HabitRPG/habitrpg into difficulty