Fixes issue with metrics tab click handlers
  Click handlers were not getting attached to DOM elements in some cases on slow running machines.
  Added logic to attach handlers when elements are ready.

Added 2 buttons on metrics tab:
  Download Subsection Data for downloading to csv.
  Download Problem Data for downloading to csv.
Merge pull request #3584 from edx/dcadams/metrics_tab_download_csv

Metrics tab - fix click handlers and add download buttons