Assessment student data report (#1594).

Replaces old problemset student data report with report for new exam
types.
Addendum to #1594

Tweaked the Assessment Student Data report. If the human_name for the
field is empty show the field name instead.
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 pull request #1685 from Stanford-Online/dca_production_copy3

Assessment reports -> production branch