Add grayscale to featured images

[crisbusquets]

uiFromMars' identity is a bit confusing. Adding a filter that changes the thumbnails to a grayscale should be enough.

[crisbusquets]

Doing it via CSS / SVG filters means that the og:image when sharing the post on social will still have color. The images need to be updated manually.

And perhaps the solution is not "grayscale", but removing the blue-ish background some of them have and turn into something black.