Fixes some stuff in metrics tab
  Fixes some 500s after opaque-keys
  Adds a test which would have caught the 500s
  Fixes Transifex problem on the string % of students
Merge pull request #3871 from edx/dcadams/fix_metrics_tab_after_opaque_keys

Fixes some stuff for metrics tab
Fix jump_to_id links returning 404 for private items in preview
Add test case for jump_to_id link resolution of draft items
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