Support expressions in SEO and social tabs (page/site settings)

[lexoyo]

SEO and social fields in page/site settings should accept expressions, so they work identically for static and dynamic sites. This also allows removing the SEO/social "overrides" in the CMS tab.

Related

• Roadmap: https://roadmap.silex.me/posts/63
• Related: The HTML "lang" attribute of the pages should be dynamic #1648 (same pattern for lang attribute)