Editors can add or remove detailed guide translations
Detailed Guides `Contents` heading is translatable
Regenerating locale files given new translation key
CSS change to horizontally align locale options
Edition already includes Translatable module
Mark detailed guide routing as localised
Remove condition that can't be reached

`best_locale_for_edition` redundantly checked `edition.non_english_edition?`. This is the first check in `document_url` and so would hit on that line, if correct, never inside `best_locale_for_edition`.
Add more translatable text to Detailed Guides show page

Add Welsh translations since the feature is primarily for Welsh readers.
Add remaining translation files
Commence translation controller refactoring

Move the `create` method to a module that extends ActiveSupport::Concern
Refactor: move `edit` method to Translation Concern
Refactor: move `update` method to Translation Concern
Refactor: move `translation_locale` method to Translation Concern
Refactor: move `destroy` method to Translation Concern
Refactor: abstract internals of `notice_message`
Refactor: remove temporary helper method from Translation Concern
Refactor: move `index` method to Translation Concern
Refactor: consolidate filters that always run into one method
Refactor: slightly better naming. Fixing alignment
Refactor: rename `load_items` to be less generic