Improve & clarify platform and offered_by assignments

[mbertrand]

Some course runs are ingested into open with the same run ids but on different platforms. Usually these are mitx (EdX) and oll (Open Learning Library); or mitxonline and mitx where the mitx runs are actually ingested by the Micromasters ETL pipeline. This is especially confusing because ingested Micromasters programs are assigned a platform of micromasters but the platform courses and runs are assigned to the mitx platform. This was originally done because all the course data came from EdX, but not it may come from MITX Online instead. Peter suggested we could modify the micromasters courses API to indicate where a course is edx.org or mitxonline.

In general, it would be good to have a more consistent approach to assigning platform and offered_by values.