Classification permissions

Adds permission controls for classifications. 
I think we need to support all of those. For #49 we certainly need to allow a user to see all classifications in projects they own, and for supporting classroom/workshop use we need at least some members of user groups to be able to view all the classifications generated by their group. 

I'm not sure they sure all be under the same endpoint, but I didn't see another way to make it fit into the current api.