Only limit container width on issues & MRs within fixed-width container

app/assets/stylesheets/pages/issuable.scss

 // Limit MR description for side-by-side diff view
-.limit-container-width {
+.container-limited {
+  &.limit-container-width {
     .detail-page-header {
       max-width: calc(#{$limited-layout-width} - (#{$gl-padding} * 2));
       margin-left: auto;
@@ -43,6 +44,7 @@
         margin-right: auto;
       }
     }
+  }
 }
 
 .issuable-details {