Replace loading text with spinner

app/assets/stylesheets/pages/merge_conflicts.scss

@@ -242,7 +242,7 @@ $colors: (
         display: none;
       }
 
-      .loading-text {
+      .loading {
         display: block;
       }
     }
@@ -262,7 +262,7 @@ $colors: (
       }
     }
 
-    .loading-text {
+    .loading {
       display: none;
     }
   }

app/views/projects/merge_requests/conflicts/components/_diff_file_editor.html.haml

 %template{ id: "diff-file-editor" }
   %div
     .editor-wrap{ ":class" => "classObject" }
-      %p.loading-text Loading...
+      .loading
+        %i.fa.fa-spinner.fa-spin
       .editor
         %pre{ "style" => "height: 350px" }