sca sales reports depend on devportal app transitions

[Michael Nelson]

The helper functions:

* devportal.stats.gather_sales_report_data,
* devportal.stats.gather_finance_report_data,

belong in the software-center-agent application, as they report on subscriptions to applications (used to be devportal.Application, but now sca.PublishedApplication).

These two functions could almost be moved to sca, along with the devportal.Application.stats call-site, except that they currently depend on state-transitions of devportal.Application objects. AFAICT, this is just historical.

sca should track itself the periods when it's PublishedApplications are exposed, and when they are removed, so these reporting functions can be completely within sca. The data from these reports may be used by devportal, if it's decided that the UI for accessing the information will remain there, but that should be the only dependency (and that eventually via an API call for the data).