extract User.enrollment_conditions from user.rb, refs CAT-147

they're not relevant to *a* User, and they're needed by things other than
User.reflections.

test plan:
n/a, just refactoring to a (hopefully) better spot. existing and new specs
should cover all of this

Change-Id: I66ab5314582113ac549af5662bcf08a8ca5df580
Reviewed-on: https://gerrit.instructure.com/35082
Reviewed-by: Dave Donahue <ddonahue@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>