[#342] Change race to apply senses & movement dynamically

[arbron]

Allows values and units on movement & senses to be null. Any null values will be replaced by data in race if a character has one.

Adds a migration that modifies any 0s in those fields to null, as long as the actor hasn't already been changed for 2.4.0 based on the built-in _stats. This will allow those fields to be explicitly set to 0 without the risk of being clobbered in future migrations.

[Fyorl]

Looks pretty good to me, thanks for doing this work, Jeff. I had one point of discussion about inheriting creature type. It would be good to decide now before we release as it will be harder to change later.

[arbron]

@Fyorl I've made it so type is only defined by race, and I've linked the configure button on the character sheet so it edits the value directly on the race item, so it is still easy to modify.

[Fyorl]

Looks good, this work is coming together nicely, thanks Jeff