Missing SEO metadata

[stefanb]

TODO:

• meta title and description with sane hardcoded defaults
• opengraph title and description with sane hardcoded defaults
• meta title and description localized in runtime with translations
• opengraph title and description localized in runtime with translations
• canonical links pointing to same URL but on production hostname zdravniki.sledilnik.org with all query string and hash striped (remove everything after ? or # inclusive
• hreflang links between same pages in different languages (language part of the url replaced with target language, cleaned of varying parts as above)

[jalezi]

@stefanb on branch feat/react-helmet is simple example.

[stefanb]

Until this is fully implemented I added a basic static canonical link pointing to production site homepage in 0ddc469

This should be improved to set the path part (language, doctor slug...) dynamically when navigating the site