Change sidebar links and add some work filters. This allows us to simplify worksubcategories a bit.
Restructure venue and work models to belong to users not categories. Remove the worksubcategory concept and make parent and children workcategories.
Remove images.
Parent_ids and workcategory_ids should be set to nill if workcategory is deleted.
Improve Activity create methods to make sure each work is in one place at a time. Also clean up clients and venues.