Update milestone title spacing with utility classes

app/assets/stylesheets/pages/milestone.scss

@@ -178,15 +178,6 @@ $status-box-line-height: 26px;
 
 .milestone-detail {
   border-bottom: 1px solid $border-color;
-
-  > h2 {
-    margin: 0;
-  }
-
-  .description {
-    border-top: 1px solid $border-color;
-    padding: $gl-padding 0 0;
-  }
 }
 
 @include media-breakpoint-down(xs) {

app/views/shared/milestones/_description.html.haml

 .detail-page-description.milestone-detail
-  %h2.title{ data: { qa_selector: "milestone_title_content" } }
+  %h2.gl-m-0{ data: { qa_selector: "milestone_title_content" } }
     = markdown_field(milestone, :title)
 
   - if milestone.try(:description).present?
     %div{ data: { qa_selector: "milestone_description_content" } }
-      .description.md
+      .description.md.gl-px-0.gl-pt-4.gl-border-1.gl-border-t-solid.gl-border-gray-100
         = markdown_field(milestone, :description)