* Use relative_url and absolute_url where possible
Drops the `contains "://"` check, adopt Jekyll 3.7
Ref: https://github.com/mmistakes/minimal-mistakes/pull/2385#issuecomment-579882236
* One more unneeded {% assign %}
* Remove one more assign as noted by mmistakes
* Consolidate 4 more captures
* Consolidate an extra assign on "active" class
Where it makes sense replace asset paths and navigation related paths with `relative_url` filter.
Leave SEO related `<head>` elements and social sharing links as `absolute_url`.
Fixes#1588
