Make titles, descriptions translatable systematically

• Partial templates use titles and descriptions when present, and use title and description templates otherwise
• Use a title and a description as both the site's and the home page's
• Related to RSS:
  • Make sections output only HTML
  • Use correct language codes
• Make the breadcrumb work like before

Now if an announcement gets the plasma layout, besides the file name, only date field is needed in the frontmatter to infer other information.