Unnecessary creation of UserPortletAssignmentMapping for each user

[davisagli]

plone.app.portlets/plone/app/portlets/dashboard/__init__.py

Line 36 in bf73e65

manager = category[userid] = UserPortletAssignmentMapping(

creates a new persistent UserPortletAssignmentMapping for each dashboard portlet manager every time a new user is added.

It should be possible to avoid this and create it lazily if they actually assign portlets to their dashboard.