Redesign post cards on frontpage

• Use default font instead of Hermann
• Reduce size by removing the image and some metainformation

Relates to #47 (closed) and !46 (closed)

_includes/cards/post_card.html

+{%- comment -%}
+
+This is a card displaying post title and date.
+Parameters:
+post: The post which sould be displayed on the card
+
+{%- endcomment -%}
+
+<div class="card {{ include.class }} bg-light">
+    <div class="card-header">
+        <span class="badge badge-secondary">
+            <i class="fas fa-calendar-day" aria-hidden="true"></i>
+            {{ include.post.data | date: "%d.&thinsp;%b.&thinsp;%y" }}
+        </span>
+    </div>
+    <div class="card-body">
+        <a href="{{ include.post.url | relative_url }}" class="stretched-link">
+            <h4 class="card-title">{{ include.post.title }}</h4>
+        </a>
+    </div>
+</div>