Adding an additional check to avoid donations showing up for microsites.

Donations are showing up for people newly enrolled on microsites (including for professional ed courses). The prof ed  courses on the microsites are course_mode "honor", but more importantly, we do not want to request donations on any microsites.

So this solution is straight forward. Adding a check when displaying the donations button to avoid displaying it for microsites. 

@andywaldrop @griffresch @wedaly @chrisndodge 

Verifying locally.
It's possible. I think we'd want to update the CourseMode model to have some must_pay boolean. We'd not only have to check if a single course mode doesn't have a price, but if all course modes for a course have a minimum price.