Migrate fa-spinner to spinner in single file diff js.

app/assets/javascripts/single_file_diff.js

@@ -9,7 +9,7 @@ import initImageDiffHelper from './image_diff/helpers/init_image_diff';
 import syntaxHighlight from './syntax_highlight';
 
 const WRAPPER = '<div class="diff-content"></div>';
-const LOADING_HTML = '<i class="fa fa-spinner fa-spin"></i>';
+const LOADING_HTML = '<span class="spinner"></span>';
 const ERROR_HTML =
   '<div class="nothing-here-block"><i class="fa fa-warning"></i> Could not load diff</div>';
 const COLLAPSED_HTML =