[Rubocop] Ensure all GrapeApis have a success response defined

Overview

To ensure consistency across our Grape API definitions, we should ensure that all grape endpoints define a success response.

Implementation plan

• Add a new rubocop which checks all API endpoints for a success field
• For all instances where this cop fails, either add a detail field (where its content is obvious), or create an issue for the product team in question to add it.