Activity date is being set to NOW instead of to Need date in some cases

There are two issues here:

1. If a user signs up for the flexible need, or if an admin adds a contact to the flexible role via the Assign UI (i.e., to the Available Volunteers pool), the activity uses NOW as its date. We probably want to default this to the time of the event rather than to NOW.

2. In the Assign UI, when a volunteer is dragged from one shift to another, the activity date is not updated.