seo: noindex changes / og: meta-tags to seo module

[mworrell]

Description

If a page should not be indexed, then do not add the hreflang links to the head.
Also:

• set the 'nofollow' in the http response headers and html head.
• move the FB "og:" meta-tags to the seo module (no need anymore to enable mod_facebook to add those)
• extra blocks around the SEO noindex tags, for easier overrule
• Fallback to the page_home depiction for the "og:image" url
• Fallback to the page_home seo_desc for the SEO description

Checklist

• documentation updated
• tests added
• no BC breaks