Add visibility flag in the blocks API

This need was mentioned here: https://github.com/edx/edx-mobile-marketplace-ios/pull/100
The block API needs to add a progress visibility flag.
At the moment we completely hide progress points.

EDIT: This issue is about adding visibility flag to the blocks API to hide progress points. From the above linked PR:

We'd hide the progress points when the descendants of a subsection aren't available under this PR: https://github.com/edx/edx-mobile-marketplace-ios/pull/99. It turns out, in the case of the verified track, the server is returning all the descendants, and the instructor still has an option to hide the progress points.

At the moment, the server is not returning that information in the blocks API, so for the time being, to address the CR, we are hiding progress points altogether. We will bring them back when the server starts sending the progress hide/show boolean in the blocks API.

Start by reading the linked pull requests 100 and 99, then locate the iOS blocks API models and progress-point handling. Confirm where the API response is represented and how the visibility value should control progress points; done means the blocks API exposes the flag and the app uses it to hide or show progress appropriately.