Extend the actor data model with a new object attributes.concentration for concentration related properties

[krbz999]

Related to #3028.

Akin to initiative, I strongly believe a new property in the system model would be the way forward rather than using flags.dnd5e.

Needed properties include:

• a field for bonuses to concentration.
• a boolean property for whether the actor has advantage.
• like initiative, include a string field to decide what ability the actor uses for concentration.
• a boolean property for whether the actor can concentrate on an item at all (some effects leave an actor unable to concentrate, for example Rage). This could also be a property in DND5E.conditionEffects instead.
• A property (preferrably a string) that determines the minimum the actor rolls on the d20 (see: Circle of Stars druid, Dragon Constellation).