feat: move some app model logic to wuttjamaican

previously the "canonical" way to get the model was via
config.get_model() - but now the canonical way is simply app.model

note that for now, even though wuttjamaican provides a default model,
the rattail model does not (yet?) inherit from it. also since
wuttjamaican does not yet support versioning, that feature remains in
rattail only for now.

various legacy things have been deprecated and will warn if used by
other apps.

Initial narrative docs for Configuration

not really complete, but gosh let's get something in place