Migrate '.fa-spinner' to '.spinner' within 'app/views/projects/merge_requests'

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

The directory mentioned in this issue's title still makes use of .fa-spinner directly and by calling = spinner or icon('spinner spin) 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.