Add option to dowload submissions per role

I need a way filter user submissions per role when downloading submissions. This is needed when the site has one default questionnaire, which is answered by people from many different user groups simultaneously. The best way to get a list of submissions for one specific group is to have different roles for each of the groups and then filter the submissions based on that role.

The attached patch adds this functionality. The patch is quite straightforward and shouldn't cause any unwanted side effects. I really hope this patch will be accepted as part of the next version.