Get active student count from new analytics data API

Disabled by default.
Uses ModuleStoreEnum in is_xblock_visible_to_students
Merge pull request #4234 from rocha/rocha/analytics-active-count

Get active student count from new analytics data API
Merge pull request #4259 from edx/benmcmorran/fix-xblock-visibility

Uses ModuleStoreEnum in is_xblock_visible_to_students