R1:2588b4fac0c1

Added initial class for displaying invisible chars

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…