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

app/javascript/styles/mastodon-light/variables.scss

@@ -39,3 +39,5 @@ $account-background-color: $white !default;
 @function lighten($color, $amount) {
   @return hsl(hue($color), saturation($color), lightness($color) - $amount);
 }
+
+$emojis-requiring-inversion: 'chains';