Fix enrollment data backfill Django management command

[johnbaldwin]

Update backfill_figures_enrollment_data Django management command to use new workflow

• With basic test coverage now
• Moved figures.backfill to figures.pipeline.backfill because that's where it belongs

At the core, this is really just a convenience wrapper around figures.tasks.backfill_enrollment_data_for_course

As we use it, we'll iterate on CLI to improve usability.