update changes

app/views/projects/blob/viewers/_loading.html.haml

-.text-center.prepend-top-default.gl-mb-3
+.text-center.gl-mt-3.gl-mb-3.
   .spinner.spinner-md.qa-spinner.mt-1

app/views/projects/blob/viewers/_sketch.html.haml

 .file-content#js-sketch-viewer{ data: { endpoint: blob_raw_path } }
-  .text-center.prepend-top-default.gl-mb-3.js-loading-icon
+  .text-center.gl-mt-3.gl-mb-3.js-loading-icon
     .spinner.spinner-md.mt-1

app/views/projects/blob/viewers/_stl.html.haml

 .file-content.is-stl-loading
-  .text-center.prepend-top-default.append-bottom-default#js-stl-viewer{ data: { endpoint: blob_raw_path } }
+  .text-center.gl-mt-3.gl-mb-3.#js-stl-viewer{ data: { endpoint: blob_raw_path } }
     .spinner.spinner-md.mt-1
-  .text-center.prepend-top-default.gl-mb-3.stl-controls
+  .text-center.gl-mt-3.gl-mb-3.stl-controls
     .btn-group
       %button.btn.btn-default.btn-sm.js-material-changer{ data: { type: 'wireframe' } }
         Wireframe