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