The `may_certify` function is used to determine whether a learner should
be able to see that their certificate is ready. It is therefore entirely
in the certificates domain. This remove may_certifiy and
may_certify_for_course from the xmodule and course_overview
apps and into the certificates app. The xmodule may_certify was not called
outside of tests prior to this, and the other's were easily moved.
In order to avoid circular imports, this also moved
certificate_info_for_user out of the model code and into an API
d16c9a804c