H5P editor: upload/download content-type translations

Find a mechanism to let users upgrade language strings (without to upgrade the whole content-type library).

List of tasks

• For each H5P content-type library, let admins download the existing translation files.
• For each H5P content-type library, let admins upload translation files:
  • If existing, they will overwrite it.
  • If not, the translation will be created.