Fixed image discussion styling

Fixes the styling of image discussions after the discussion styling
changes.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54110

app/assets/stylesheets/framework/files.scss

@@ -415,7 +415,6 @@ span.idiff {
 }
 
 .preview-container {
-  height: 100%;
   overflow: auto;
 
   .file-container {

app/assets/stylesheets/pages/diff.scss

@@ -842,11 +842,15 @@
     background-repeat: repeat;
   }
 
-  .diff-file-discussions + .discussion-form::before {
-    width: auto;
-    margin-left: -16px;
-    margin-right: -16px;
-    margin-bottom: 16px;
+  .diff-file-discussions + .discussion-form {
+    padding: $gl-padding;
+
+    &::before {
+      width: auto;
+      margin-left: -$gl-padding;
+      margin-right: -$gl-padding;
+      margin-bottom: $gl-padding;
+    }
   }
 
   .notes {

app/assets/stylesheets/pages/notes.scss

@@ -433,14 +433,8 @@ $note-form-margin-left: 72px;
   }
 
   .discussion-notes {
-    &:not(:first-child) {
-      border-top: 1px solid $white-normal;
-      margin-top: 20px;
-    }
-
     &:not(:last-child) {
-      border-bottom: 1px solid $white-normal;
-      margin-bottom: 20px;
+      margin-bottom: 0;
     }
 
     .system-note {