Improve rendering of emoji which do not contrast with background (#13772)

* Refactor list of emoji requiring added outlines so that it can be theme-specific

* Split inaccessible emoji to emoji requiring an outline and ones that can be inverted

* Drop the “silouhettes” from black emoji as they seem to have changed color

* Add inaccessible emojis list for the light theme

* Use bordered emoji variant instead of unreliable CSS