* Survey response startDate stored twice
startDate was being stored both in a column in Survey_response and also inside the
serialised responseJSON. Consolidated to column and moved startDate methods to
Survey.pm where they are actually used. Was not causing errors because both copies
were initialised to "now" at response creation time, and then never changed (this is also
why we don't need any repair code to fix existing survey repsonses in the wild).
* Survey ExpireIncompleteSurveyResponses Workflow Activity not enabled
The only time you'd actually want to modify startDate is when you're trying to simulate
response expiry in test code, which is why I found the above bug when I was writing the
missing test suite for ExpireIncompleteSurveyResponses. Along with test suite, added
upgrade code to enable workflow activity and add it to the Daily Maintenance Tasks
workflow. Also made minor fixes to the workflow activity, such as making sure it uses
the correct isComplete code.
