A11Y: Set role=presentation if alt attr is missing (#18546)

This applies to all images posted that do not have a user description.

app/assets/javascripts/pretty-text/addon/allow-lister.js

@@ -173,6 +173,7 @@ export const DEFAULT_LIST = [
   "iframe[allowfullscreen]",
   "iframe[allow]",
   "img[alt]",
+  "img[role]",
   "img[height]",
   "img[title]",
   "img[width]",

app/assets/javascripts/pretty-text/addon/engines/discourse-markdown-it.js

@@ -253,7 +253,13 @@ function renderImageOrPlayableMedia(tokens, idx, options, env, slf) {
     }
   }
 
-  token.attrs[token.attrIndex("alt")][1] = altSplit.join("|");
+  const altValue = altSplit.join("|").trim();
+  if (altValue === "") {
+    token.attrSet("role", "presentation");
+  } else {
+    token.attrSet("alt", altValue);
+  }
+
   return slf.renderToken(tokens, idx, options);
 }