A11Y: Set an empty `alt` tag on category logos (PR #12830)

We don’t have the ability to set alt or aria- tags on category logos, and until we do we should consider the logo decorative. Here I’m setting an empty alt tag so screen readers ignore them. When an alt tag isn’t set some screen readers simply read “graphic” to the user, which is useless in this context!

Some discussion here: A11y for Categories - #3 by awesomerobot - support - Discourse Meta


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