add cached attr for projects

Plausibly we'll want to have "active_projects" at some point, but for now we can not sweat it.  We'll want this for eventually letting `++` put work into projects.  But for now, we might just want this for project listing.  (We could also use this for taking Issue structs and resolving project.id to project.name on our own.  This will be a question of how we use GraphQL for this kind of thing.)