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/gallery

@@ -19,33 +19,17 @@
 <figure class="{{ gallery_layout }} {{ include.class }}">
   {% for img in gallery %}
     {% if img.url %}
-      <a href=
-        {% if img.url contains "://" %}
-          "{{ img.url }}"
-        {% else %}
-          "{{ img.url | relative_url }}"
-        {% endif %}
-        {% if img.title %}title="{{ img.title }}"{% endif %}
-      >
-        <img src=
-          {% if img.image_path contains "://" %}
-            "{{ img.image_path }}"
-          {% else %}
-            "{{ img.image_path | relative_url }}"
-          {% endif %}
-          alt="{% if img.alt %}{{ img.alt }}{% endif %}">
+      <a href="{{ img.url | relative_url }}"
+        {% if img.title %}title="{{ img.title }}"{% endif %}>
+          <img src="{{ img.image_path | relative_url }}"
+               alt="{% if img.alt %}{{ img.alt }}{% endif %}">
       </a>
     {% else %}
-      <img src=
-        {% if img.image_path contains "://" %}
-          "{{ img.image_path }}"
-        {% else %}
-          "{{ img.image_path | relative_url }}"
-        {% endif %}
-        alt="{% if img.alt %}{{ img.alt }}{% endif %}">
+      <img src="{{ img.image_path | relative_url }}"
+           alt="{% if img.alt %}{{ img.alt }}{% endif %}">
     {% endif %}
   {% endfor %}
   {% if include.caption %}
     <figcaption>{{ include.caption | markdownify | remove: "<p>" | remove: "</p>" }}</figcaption>
   {% endif %}
-</figure>
+</figure>