Migrate Arc's UD model layer into LocoKit

[sobri909]

Arc keeps an extra layer of activity type ML models than LocoKit. The extra layer being the user's own personal models, based on their own type confirmations / corrections.

• In LocoKit the tree is GD2 -> GD1 -> GD0 (neighbourhood -> city -> world).
• In Arc the tree is UD2 -> GD2 -> GD1 -> GD0 (personal neighbourhood -> shared neighbourhood -> city -> world).

So that UD2 layer needs to be migrated into LocoKit.

This will also open the door for doing custom activity types directly in LocoKit (and also in Arc).

Todos

• Migrate UserActivityType upstream
• Migrate UserActivityTypeClassifier upstream
• Migrate UserTimelineClassifier upstream