Pre & Next Links for Ghost_Head

closes #685
- Now that we have a ‘pagination’ meta object, we can implement
SEO-friendly `next` and `prev` ref links in `<head>`.
- This implementation works uniformly for anything that supports
pagination in the current schema (posts, tags, authors)
- Regex should make the implementation future-proof for additional
pagination
Merge pull request #3817 from felixrieseberg/iss685

Prev & Next Links for Ghost_Head