Prevent users from assigning issues to themselves

Currently, even without the “assign and be assigned project issues” permission, authenticated users can assign issues to themselves and unassign them from the current user. The attached patch adds a new permission, “assign project issues to self”, that controls this behavior. Without this permission (and without “assign and be assigned project issues”), issue creation and comment add shows a fixed string for assignee rather than a dropdown box (a new issue is created unassigned).

After installing this patch, be sure to give the new permission to the desired roles; if you want the pre-patch behavior, give it to Anonymous and Authenticated users.