add a data export button

Created by: John-B

Data subjects are entitled to receive the data we hold on them. A contact export does not include activity data, which must be generated seprately under Reports, as far as I can see. A button offering data export which includes all data (apart from fields we choose to exclude) and all activities for a data subject would be very useful.

The data export would presumably be in CSV, and a PDF option woud be nice to have.