Summary: Fixes T11586. First pass at a class for displaying invisible characters. Still need to: - Write a couple unit tests - Add some styling to the .invisible-special spans - Actually start using the class when displaying form errors to users Currently this makes the string `"\nab\x00c\x01d\te\nf"` look like: {F1812711} Test Plan: Unit tests all pass and run in <1ms: {F1812998} Reviewers: epriestley, #blessed_reviewers, chad Reviewed By: epriestley, #blessed_reviewers Subscribers: Korvin, epriestley, yelirekim Maniphest Tasks: T11586 Differential Revision: https://secure.phabricator.com/D16541