A11Y: onebox avatars/images are decorative, so screenreaders can skip (#33572)

Adding `alt=""` to tell screenreaders to skip these, we don't reliably
have alt content available for onebox images and they tend to be
decorative, so better to omit.


<img width="738" height="151" alt="learn.microsoft.com onebox with the
link whats new in C# 11, there's a purple C# badge to the left of the
link"
src="https://github.com/user-attachments/assets/15bdc830-683f-4c69-a11b-9d54c66fa627"
/>


We already do this in some oneboxes, so this covers some more cases
including the generic.

lib/onebox/templates/_layout.mustache

@@ -1,7 +1,7 @@
 <aside class="onebox {{subname}}" data-onebox-src="{{link}}">
   <header class="source">
     {{#favicon}}
-      <img src="{{favicon}}" class="site-icon"/>
+      <img src="{{favicon}}" class="site-icon" alt=""/>
     {{/favicon}}
 
     {{#article_published_time}}

lib/onebox/templates/allowlistedgeneric.mustache

@@ -1,4 +1,4 @@
-{{#image}}<img width="{{thumbnail_width}}" height="{{thumbnail_height}}" src="{{image}}" class="thumbnail"/>{{/image}}
+{{#image}}<img width="{{thumbnail_width}}" height="{{thumbnail_height}}" src="{{image}}" class="thumbnail" alt=""/>{{/image}}
 
 <h3><a href="{{link}}" target="_blank" rel="noopener">{{title}}</a></h3>
 

lib/onebox/templates/preview_error_onebox.mustache

@@ -1,6 +1,6 @@
 <aside class="onebox preview-error">
   {{#favicon}}
-    <img src="{{favicon}}" class="site-icon"/>
+    <img src="{{favicon}}" class="site-icon" alt=""/>
   {{/favicon}}
   <a href="{{link}}" target='_blank' rel='noopener noreferrer'>{{title}}</a>
   <article class="onebox-warning-message">

lib/onebox/templates/threadsstatus.mustache

@@ -1,11 +1,11 @@
-{{#avatar}}<img src="{{avatar}}" class="thumbnail onebox-avatar">{{/avatar}}
+{{#avatar}}<img src="{{avatar}}" class="thumbnail onebox-avatar" alt="">{{/avatar}}
 <h4><a href="{{link}}" target="_blank" rel="noopener">{{title}}</a></h4>
 <div class="threads-screen-name"><a href="{{link}}" target="_blank" rel="noopener">@{{screen_name}}</a></div>
 
 <div class="thread-contents">
   <span class="thread-description">{{{description}}}</span>
   {{#image}}
-    <div class="scale-images"><img src="{{image}}"></div>
+    <div class="scale-images"><img src="{{image}}" alt=""></div>
   {{/image}}
 </div>
 

lib/onebox/templates/twitterstatus.mustache

@@ -1,4 +1,4 @@
-{{#avatar}}<img src="{{avatar}}" class="thumbnail onebox-avatar">{{/avatar}}
+{{#avatar}}<img src="{{avatar}}" class="thumbnail onebox-avatar" alt="">{{/avatar}}
 <h4><a href="{{link}}" target="_blank" rel="noopener">{{title}}</a></h4>
 <div class="twitter-screen-name"><a href="{{link}}" target="_blank" rel="noopener">@{{screen_name}}</a></div>