Switch v2 creature damage type to use model values.

[augustjohnson]

Currently:

def damage_type_field():
return models.CharField(
null=True,
max_length=20,
choices=DAMAGE_TYPES,
help_text='What kind of damage this attack deals.'
)

Choices should be based on the model objects in the Damage Type model.

[augustjohnson]

Addressed in #425

[augustjohnson]

This has been merged.