Assessment Summary report which replaces Problem set summary report
(#1595)

As it turns out, this was quite simple as it is a report of a single
assessment with same format as the Combined Summary Assessments
report.

Also, includes some final cleanup of the old problem set reports that
are no longer required.
Merge branch 'master' of http://github.com/Stanford-Online/class2go into dca_new_working
Merge pull request #1670 from Stanford-Online/dca_new_working

Assessment Summary report #1595 (replaces problemset summary report)