Send transaction type for donations and verified certificates

[ECOM-543](https://openedx.atlassian.net/browse/ECOM-543): Donations analytics

Reviewers: @dianakhuang @stephensanchez 
@stephensanchez @dianakhuang I've updated this to send the course mode slug directly instead of trying to parse it into verified / prof ed.  I think this is safer in the long term.  Thoughts?