Summary, assignee and planned end date required

… on task group modal.
Corrected validation logic for contact.
Fix TaskGroup validation for end_date and assignee

The logic in this commit should work after a separate bug is fixed. I’m
checking in this code now. Anze and Brad will work to resolve the other
bug. The bug I’m referring to was introduced in commit
8a5f600c65b7b45fd34bf8a7631961a6d5a19638 to resolve a bit of a circular
loop related to autocomplete in search.
Reference to start date should be to end date
Merge pull request #1494 from reciprocity/feature/workflow-task-group-fixes

Feature/workflow task group fixes