Use relative_url and absolute_url where possible (#2387)

* 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

_includes/archive-single.html

@@ -14,13 +14,7 @@
   <article class="archive__item" itemscope itemtype="https://schema.org/CreativeWork">
     {% if include.type == "grid" and teaser %}
       <div class="archive__item-teaser">
-        <img src=
-          {% if teaser contains "://" %}
-            "{{ teaser }}"
-          {% else %}
-            "{{ teaser | relative_url }}"
-          {% endif %}
-          alt="">
+        <img src="{{ teaser | relative_url }}" alt="">
       </div>
     {% endif %}
     <h2 class="archive__item-title" itemprop="headline">