Always use HTML attributes for avatar size (#31509)

Many avatars were rendered in HTML with certain width/height but then
resized again in CSS. This was pointless so I removed all these cases
and made the HTML size match the previous render size.

Also did a few CSS cleanups in the tribute rendering:

<img width="648" alt="image"
src="cb2fafb3-5e20-46e9-814f-07df20038beb">

web_src/js/features/tribute.js

@@ -36,7 +36,7 @@ function makeCollections({mentions, emoji}) {
       menuItemTemplate: (item) => {
         return `
           <div class="tribute-item">
-            <img src="${htmlEscape(item.original.avatar)}" class="tw-mr-2"/>
+            <img src="${htmlEscape(item.original.avatar)}" width="21" height="21"/>
             <span class="name">${htmlEscape(item.original.name)}</span>
             ${item.original.fullname && item.original.fullname !== '' ? `<span class="fullname">${htmlEscape(item.original.fullname)}</span>` : ''}
           </div>