Add support for bulk task updating

This allows org admins to update tasks after they've been submitted via
the bulk uploader, provided they originally included a "sync" value.
This rejects updates to title, description, time to complete, and beginner
if a task is active (being worked on by a student), but allows updating
metadata (mentors, tags, task types) any time.

Fixes issue 2391

Change-Id: I88617b1a42978a4c3500dd6fd44ff054a1f89493
2 files changed