Fix date in ActivityWidget and remove unnecessary string conversion

Fix date in ActivityWidget and remove unnecessary string conversion

The local date and time value was converted into a string, just to be converted
into another string, to be converted to a value once again, returning zero as
the result. This caused the widget to always display "now".

Looks like this was a simply copy and paste mistake from this line in
ActivityListModel::slotActivitiesReceived:

a._dateTime = QDateTime::fromString(json.value("date").toString(), Qt::ISODate);

Signed-off-by: Michael Schuster <michael@schuster.ms>