Hide the alt text for missing IdP logos

The current display of alt text for missing IdP logos is a bit jarring.

While having the text available is good practice for a11y, it shouldn't be displayed normally.

One way of achieving this is to set color: transparent; on the img tags.

Setting the font-size: 0; has the side-effect of influencing the em unit, so that would require us to switch to rem.

mentioned in merge request !2 (merged)

closed