fix: cleanup old code for "record changes" session feature

had some tests failing, and one thing led to another.. apparenty we
still had support for some pretty old sqlalchemy so no need to keep
that around.

feat: refactor config/extension, session logic per wuttjamaican

this leverages wuttjamaican for a few more things perhaps, but mostly
this is to organize the code in a more logical way based on
conventions used in wuttjamaican.

feat: move most of auth handler logic to wuttjamaican

also deprecate the `rattail.db.auth` module

fix: remove duplicate method for `AppHandler.load_object()`

that is now defined in wuttjamaican

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.

fix: revert back to setup.py + setup.cfg

apparently with python 3.6 things "mostly" work but then they break if
any specified dependencies have a dot in the name. which in this
project, is the case for `flufl.bounce`

so until we drop python 3.6 support, we cannot use pyproject.toml here

build: fix project license attribute

avoid including the full license text