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/sidebar.html

@@ -4,13 +4,8 @@
   {% if page.sidebar %}
     {% for s in page.sidebar %}
       {% if s.image %}
-        <img src=
-        {% if s.image contains "://" %}
-          "{{ s.image }}"
-        {% else %}
-          "{{ s.image | relative_url }}"
-        {% endif %}
-        alt="{% if s.image_alt %}{{ s.image_alt }}{% endif %}">
+        <img src="{{ s.image | relative_url }}"
+             alt="{% if s.image_alt %}{{ s.image_alt }}{% endif %}">
       {% endif %}
       {% if s.title %}<h3>{{ s.title }}</h3>{% endif %}
       {% if s.text %}{{ s.text | markdownify }}{% endif %}
@@ -21,4 +16,4 @@
     {% endif %}
   {% endif %}
   </div>
-{% endif %}
+{% endif %}