Add a block template with flags and pretty url

This pull request adds a new template called `seo_flags`, similar to
the `flags` template, except that:
- the url of the flags are the final url of the translated pages
- the `ccm-multilingual-switch-language-flags-label` is printed out
  only if the $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).
Minor change
Add link rel="alternate" hreflang=".." href=".."

This commit adds the possibility to add the
```html
<link rel="alternate" hreflang=".." href=".." />`
tags to the pages to signal search engines the alternate versions of a
page. This is enabled by defining MULTILINGUAL_ADD_ALTERNATE_HREFLANG
to true.
Removed files from other pull request
Merge pull request #16 from mlocati/add-link-hreflang

Add link hreflang