Update schema to reflect re-usable error_template_attributes, descriptions, hints, and sorting

app/models/error_template.rb

@@ -1,3 +1,4 @@
 class ErrorTemplate < ActiveRecord::Base
   belongs_to :execution_environment
+  has_and_belongs_to_many :error_template_attributes
 end

app/models/error_template_attribute.rb

@@ -1,3 +1,3 @@
 class ErrorTemplateAttribute < ActiveRecord::Base
-  belongs_to :error_template
+  has_and_belongs_to_many :error_template
 end

app/models/execution_environment.rb

@@ -11,6 +11,7 @@ class ExecutionEnvironment < ActiveRecord::Base
   has_many :exercises
   belongs_to :file_type
   has_many :hints
+  has_many :error_templates
 
   scope :with_exercises, -> { where('id IN (SELECT execution_environment_id FROM exercises)') }