Add role checks for ProgrammingGroup

So far, a programming group is always considered to be a learner. While we could, technically, consider it to be a teacher or an admin if all members have this role, this behaviour might have unintended side effects.

app/models/programming_group.rb

@@ -28,6 +28,18 @@ class ProgrammingGroup < ApplicationRecord
     false
   end
 
+  def learner?
+    true
+  end
+
+  def teacher?
+    false
+  end
+
+  def admin?
+    false
+  end
+
   def self.parent_resource
     Exercise
   end