Detailed guides translations

Many transitioning agencies need to provide translated information to users and the format most suited to this is a Detailed Guide. This adds the ability to add translations for Detailed Guides.

https://www.agileplannerapp.com/boards/173808/cards/5260

Also attempting to factor out commonalities in the translation controllers by creating a concern that gets included.
Would `load_translatable_item` be a better name for this method?