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.

This commit updates lms/templates.

These keys are now objects with a limited interface, and the particular
internal representation is managed by the data storage layer (the
modulestore).

For the LMS, there should be no outward-facing changes to the system.
The keys are, for now, a change to internal representation only. For
Studio, the new serialized form of the keys is used in urls, to allow
for further migration in the future.

Co-Author: Andy Armstrong <andya@edx.org>
Co-Author: Christina Roberts <christina@edx.org>
Co-Author: David Baumgold <db@edx.org>
Co-Author: Diana Huang <dkh@edx.org>
Co-Author: Don Mitchell <dmitchell@edx.org>
Co-Author: Julia Hansbrough <julia@edx.org>
Co-Author: Nimisha Asthagiri <nasthagiri@edx.org>
Co-Author: Sarina Canelake <sarina@edx.org>

[LMS-2370]

  Clicking on any of the bars displays a list of students for that
  particular action (either opened the subsection or attempted the
  problem).
  Students are listed for the sub-sections.
  Students, grade and percent are listed for the problems.
  The on-screen list displays only the first 250 students with
  an overflow message if there are more students than that.
  The csv download lists all students.

Metrics tab shows student data:
  -Count of students opened a subsection
  -Grade distribution per problem

for each section/subsection of the course.

Implemented for both the old and beta dashboard
Controlled by a feature flag 'CLASS_DASHBOARD'
Data is aggregated across all students
Aggregate data computed from courseware_studentmodule