Localization support

[ForNeVeR]

There should be default localization selected by the browser headers. Also there should be a feature to forcibly set the site language.

If any material is unavailable in the selected language, user should get a possibility to select another language.

Plan

• Add the English variants of all the main pages
• Change the canonical URLs of every post/page to either /en/* or /ru/* addresses. Update the HTML markup accordingly
• Also update post "canonical" URLs for the purpose of commenting: set disqus_url
• When the user loads any page without /en or /ru prefix, he should be redirected to either Russian or English site according to his Accept-Language HTTP header. English should be the default
• Every page should have a language selector at the top in form of Rus/Eng letters
• If the localized page version is unaccessible, its language link should be disabled
• If a user visits a localized version of a page that is unavailable (but the page exists in another languages) - he should get a page explaining that (with an invitation to open the available version)
• (Translate the posts to English #63) Create additional task(s) to translate every post to English
• Don't forget to provide distinct and combined RSS links
• Post list should include links to posts in current language, and, for untranslated posts, links to versions in other language with a comment ("Unavailable for current language")
• Fix the compiler warnings
• Add redirects for old pages (/posts/xx_ru.html → /ru/posts/xx.html)
• Redirect from / to /en/
• Visit /1 should return 404 error (now fails on path parse)
• English page should work without any posts
• Write a post summarizing the changes