Fix module namespacing for mentionable concerns

This is needed when we switch to Zeitwerk. Module namespaces must
be consistent with the filesystem path.

lib/gitlab/background_migration/user_mentions/models/concerns/isolated_mentionable.rb

@@ -4,6 +4,7 @@ module Gitlab
   module BackgroundMigration
     module UserMentions
       module Models
+        module Concerns
           # == IsolatedMentionable concern
           #
           # Shortcutted for isolation version of Mentionable to be used in mentions migrations
@@ -98,4 +99,5 @@ module Gitlab
         end
       end
     end
+  end
 end

lib/gitlab/background_migration/user_mentions/models/concerns/mentionable_migration_methods.rb

@@ -4,6 +4,7 @@ module Gitlab
   module BackgroundMigration
     module UserMentions
       module Models
+        module Concerns
           # Extract common no_quote_columns method used in determining the columns that do not need
           # to be quoted for corresponding models
           module MentionableMigrationMethods
@@ -21,4 +22,5 @@ module Gitlab
         end
       end
     end
+  end
 end

lib/gitlab/background_migration/user_mentions/models/design.rb → lib/gitlab/background_migration/user_mentions/models/design_management/design.rb

@@ -7,7 +7,7 @@ module Gitlab
       module Models
         module DesignManagement
           class Design < ActiveRecord::Base
-            include MentionableMigrationMethods
+            include Concerns::MentionableMigrationMethods
 
             def self.user_mention_model
               Gitlab::BackgroundMigration::UserMentions::Models::DesignUserMention

lib/gitlab/background_migration/user_mentions/models/epic.rb

@@ -6,9 +6,9 @@ module Gitlab
     module UserMentions
       module Models
         class Epic < ActiveRecord::Base
-          include IsolatedMentionable
+          include Concerns::IsolatedMentionable
+          include Concerns::MentionableMigrationMethods
           include CacheMarkdownField
-          include MentionableMigrationMethods
 
           attr_mentionable :title, pipeline: :single_line
           attr_mentionable :description

lib/gitlab/background_migration/user_mentions/models/note.rb

@@ -6,7 +6,7 @@ module Gitlab
     module UserMentions
       module Models
         class Note < ActiveRecord::Base
-          include IsolatedMentionable
+          include Concerns::IsolatedMentionable
           include CacheMarkdownField
 
           self.table_name = 'notes'

lib/gitlab/background_migration/user_mentions/models/snippet.rb

@@ -6,9 +6,9 @@ module Gitlab
     module UserMentions
       module Models
         class Snippet < ActiveRecord::Base
-          include IsolatedMentionable
+          include Concerns::IsolatedMentionable
+          include Concerns::MentionableMigrationMethods
           include CacheMarkdownField
-          include MentionableMigrationMethods
 
           attr_mentionable :title, pipeline: :single_line
           attr_mentionable :description