Add seo flags

This pull request adds a new block template to `switch_language` called
`seo_flags`, similar to the `flags` template, except that:
- the destination link of the flags is the final url of the translated
  pages
- the `ccm-multilingual-switch-language-flags-label` is printed out
  only if `$label` is not empty.

This pull requests also introduces a new `InterfacePageHelper` helper
that's able to resolve the translated page of the current page taking
in account page aliases (of the current page and/or of the translated
pages).