[SD-1119] Adding default role to new user.
(fix:delete roles that still have users) [SD-1118] Add atribute to role to indicate default role
Merge branch 'feat-privilege' of https://github.com/superdesk/superdesk-server into feat-privilege
Merge remote-tracking branch 'upstream/feat-privilege' into feat-privilege
Merge remote-tracking branch 'andrew/feat-privilege' into feat-privilege
Merge remote-tracking branch 'mayur/feat-privilege' into feat-privilege

Conflicts:
	apps/users/services.py
[SD-1119] fixing the code are merge conflict.

close #389
fix(privileges): make privilege names ui friendly

SD-1124