Having specific protobuf types for different game systems using the
same rule set (e.g. all the Chronicles of Darkness games) is untenable
because protobuf does not have inheritance or mixins.
Instead, we have one generic character sheet type, with a oneof field
for the game specifics. This will be used in conjunction with the
character's game system (stored in db) to render different stuff on
the character templates.
Without this, we'd wind up having duplicate templates, a lot more code
for handling specifics of each game system, and so on.
jeff