Display correct default EULA translation for extensions/modules  (bnc#894460)

- includes small refactoring (moving the code to a separate class for better testability + small UrlHelpers improvements)

- tested in installed SLES-12 GM, `LC_ALL=de_DE.utf8 yast2 scc` then properly displays the German EULA as the default translation (instead of `en_US`).
`@licenses["en_US"] ||= license_file`