Add backend for tips and enable markdown support

app/models/file_type.rb

@@ -32,6 +32,10 @@ class FileType < ApplicationRecord
   end
   private :set_default_values
 
+  def programming_language
+    editor_mode.gsub('ace/mode/', '')
+  end
+
   def to_s
     name
   end

app/models/tip.rb

@@ -20,4 +20,9 @@ class Tip < ApplicationRecord
       "#{I18n.t('activerecord.models.tip.one')} #{id}"
     end
   end
+
+  def can_be_destroyed?
+    # This tip can only be destroyed if it is no parent to any other exercise tip
+    ExerciseTip.where(parent_exercise_tip: exercise_tips).none?
+  end
 end