Add administrators UserSegment

We need a user segment to have only administrators as recipients of both
newsletters and notifications

lib/user_segments.rb

@@ -1,5 +1,6 @@
 class UserSegments
   SEGMENTS = %w(all_users
+                administrators
                 proposal_authors
                 investment_authors
                 feasible_and_undecided_investment_authors
@@ -10,6 +11,10 @@ class UserSegments
     User.active
   end
 
+  def self.administrators
+    all_users.administrators
+  end
+
   def self.proposal_authors
     author_ids(Proposal.not_archived.not_retired.pluck(:author_id).uniq)
   end