Extend translation infrastructure to exploration metadata and skills

Currently, Oppia translates non-user-generated content via translatewiki and some user-generated content via the contributor dashboard. This project aims to extend the contributor dashboard's infrastructure to support translating exploration metadata (titles, tags) and skill (concept card) content. The solution involves building a highly extensible backend translation framework to display translation opportunities, surface reviewable suggestions, and display accepted translations to learners. Deliverables include migrating existing translation support to this new modular infrastructure (Milestone 1) and adding full support for metadata and skill content while removing legacy code (Milestone 2).