feat(avatar): Add "is-pixelated" class for avatars with styles for all browsers

[besasam]

Description This adds a new class is-pixelated to avatars to be used instead of image-rendering: pixelated since this is rendered differently in different browsers (e.g. hard eges in Chrome, but anti-aliased in Firefox). See also: https://css-tricks.com/almanac/properties/i/image-rendering/

Story and Docs are also updated accordingly.

Compatibility Should not break anything.

Caveats None that I know of.