chore: Enable Rubocop for migrations

Rubocop gives valuable information about issues in migration. New
migrations should be checked by the linter.

Author: arkirchner

.rubocop_todo.yml

@@ -22,23 +22,6 @@ Metrics/CyclomaticComplexity:
 Metrics/PerceivedComplexity:
   Max: 25
 
-# We don't want to change previous migrations...
-#
-Rails/CreateTableWithTimestamps:
-  Enabled: false
-
-Rails/BulkChangeTable:
-  Enabled: false
-
-Rails/ReversibleMigration:
-  Enabled: false
-
-Rails/NotNullColumn:
-  Enabled: false
-
-Rails/ThreeStateBooleanColumn:
-  Enabled: false
-
 # The models need to be fixed anyway
 #
 Rails/UniqueValidationWithoutIndex:

db/migrate/20140625134118_create_exercises.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class CreateExercises < ActiveRecord::Migration[4.2]

db/migrate/20140626143132_create_execution_environments.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class CreateExecutionEnvironments < ActiveRecord::Migration[4.2]

db/migrate/20140626144036_create_submissions.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class CreateSubmissions < ActiveRecord::Migration[4.2]

db/migrate/20140630093736_add_reference_implementation_to_exercises.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class AddReferenceImplementationToExercises < ActiveRecord::Migration[4.2]

db/migrate/20140630111215_add_indent_size_to_execution_environments.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class AddIndentSizeToExecutionEnvironments < ActiveRecord::Migration[4.2]

db/migrate/20140701120126_create_consumers.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class CreateConsumers < ActiveRecord::Migration[4.2]

db/migrate/20140701122345_create_users.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class CreateUsers < ActiveRecord::Migration[4.2]

db/migrate/20140702100130_add_oauth_key_to_consumers.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class AddOauthKeyToConsumers < ActiveRecord::Migration[4.2]

db/migrate/20140703070749_add_oauth_secret_to_consumers.rb

@@ -1,3 +1,5 @@
+# rubocop:disable all
+
 # frozen_string_literal: true
 
 class AddOauthSecretToConsumers < ActiveRecord::Migration[4.2]