Fix image attachments overflowing their container

This reverts commit e1b4d8f59a
and obsoletes commit c2db0e66ef
which already unset min-height in notifications where this
caused images to become effectively invisible.

Image previews are currently designed to always show the full image
scaled down as needed. With min-height though they were allowed to
take the full width even if it caused overflows in vertical direction.
This happened to look kind of fine with only easy-to-miss overflows
in the main post view if each preview row had the same amount of
columns, but creates jarring overlaps otherwise.

Fixes: #456

src/components/attachment/attachment.scss

@@ -16,7 +16,7 @@
 
   .attachment-wrapper {
     flex: 1 1 auto;
-    min-height: 200px;
+    height: 200px;
     position: relative;
     overflow: hidden;
     align-content: center;

src/components/notification/notification.scss

@@ -101,8 +101,4 @@
     color: $fallback--cBlue;
     color: var(--cBlue, $fallback--cBlue);
   }
-
-  .attachment-wrapper {
-    min-height: unset;
-  }
 }