This djangoapp was designed for talking to sailthru, in a fairly
edx.org-specific way. Nowadays, edx.org doesn't need this code and
if other installations do, it's better off as a more distinct
plugin anyway, rather than direct support in the platform.
I've moved the one signal that was still useful (calling
segment.identify() whenever user fields change) into user_authn.
And I've left the EmailMarketingConfiguration model alone for now,
but will remove that shortly. Nothing uses it as of this commit.
AA-607
DEPR-139
Removed most of the deprecated shoppingcart app, leaving just enough to allow us to cleanly remove the related database tables later. Also removed the relevant Django settings that weren't in use elsewhere.
