1) Implement the latest dataset read/write access settings from frontend to
   backend. Also updates for simultaneous read-only usage.

2) New configure options: PROTOGENI_LOCALUSER and PROTOGENI_GENIWEBLOGIN.

   The first changes the way that projects and users are treated at the
   CM. When set, we create real accounts (marked as nonlocal) for users and
   also create real projects (also marked as nonlocal). Users are added to
   those projects according to their credentials. The underlying experiment
   is thus owned by the user and in the project, although all the work is
   still done by the geniuser pseudo user. The advantage of this approach
   is that we can use standard emulab access checks to control access to
   objects like datasets. Maybe images too at some point.

   NOTE: Users are not removed from projects once they are added; we are
   going to need to deal with this, perhaps by adding an expiration stamp
   to the groups_membership tables, and using the c...