Migrate '.fa-spinner' to '.spinner' for 'app/assets/javascripts/single_file_diff.js'

The old font awesome .fa-spinner class begun being migrated to our own .spinner class early in 2019.

The file mentioned in this issue's title still makes use of .fa-spinner and needs to be migrated. This needs to happen in order to provide a consistent user experience.

If any problems are encountered while performing the migration, please raise them on the associated epic.