CKEditor: Add filter to allow empty HTML tags

FontAwesome recommends displaying its icons using <i class="fab fa-facebook"></i> (an empty i tag). However CKEditor removes empty tags.

One way around this is to enable source mode in CKEditor, add the empty i tag and then save the entity without turning off source mode (so CKEditor doesn't remove the tag). However I'm wanting to use this in a block and, because of https://github.com/backdrop/backdrop-issues/issues/1259, this workaround doen't work.

The proper solution to allow empty tags in CKEditor is to add some code to a config.js file, however I'm not sure how to do that in Backdrop without hacking core... Is there another way I can do this, or can we add a UI option in core to specify allowed empty tags?