Refactor nginx config and add SEO headers

[MrOrz]

Fixes cofacts/rumors-site#347.

• Refactor nginx config
  • extract common sections and make use of include directive
  • Consolidate rumors-site config to 2 files only
    • All staging server blocks in sites-enabled/rumors-site-staging
    • All production server blocks in sites-enabled/rumors-site; rumors-site-zh and rumors-site-en configs are all merged into rumors-site.
    • On staging we just remove sites-enabled/rumors-site, and on production we remove sites-enabled/rumors-site-staging.
• Canonical URL header
  • Use HTTP header https://developers.google.com/search/docs/advanced/crawling/consolidate-duplicate-urls#rel-canonical-link-method
  • Consolidate dup page of the same language https://developers.google.com/search/docs/advanced/crawling/managing-multi-regional-sites to (*.)cofacts.tw
• hreflang header
  • Use HTTP header https://developers.google.com/search/docs/advanced/crawling/localized-versions#expandable-2
  • Return link between cofacts.tw and en.cofacts.tw
  • We do not differentiate landing pages and content pages because
    • Differentiating them makes config more complex
    • Google document says Localized versions of a page are only considered duplicates if the main content of the page remains untranslated., which means that localizing only the templates are not punished anyway; it's considered duplicates, just like canonical URLs.
    • The same document also lists "keep the main content in a single language and translate only the template, such as the navigation and footer" as one of the example scenarios that "indicating alternate pages is recommended".