FEATURE: Support upload:// urls in img tags (#16277)

Previously, our `upload://` protocol urls were only supported in markdown image tags. This meant that our PullHotlinkedImages job was forced to convert `<img` tags to markdown. Depending on the exact syntax, this can actually cause the image to break.

This commit adds support for `upload://` inside regular HTML `<img` tags. In a future commit, we'll be able to use this to make our PullHotlinkedImages job much more robust.

Context at https://meta.discourse.org/t/152801

spec/lib/pretty_text_spec.rb

@@ -1877,6 +1877,12 @@ HTML
 
       ![upload](#{upload.short_url.gsub(".png", "")})
 
+      Inline img <img src="#{upload.short_url}">
+
+      <div>
+        Block img <img src="#{upload.short_url}">
+      </div>
+
       [some attachment](#{upload.short_url})
 
       [some attachment|attachment](#{upload.short_url})
@@ -1901,6 +1907,10 @@ HTML
         </li>
         </ul>
         <p><img src="#{cdn_url}" alt="upload" data-base62-sha1="#{upload.base62_sha1}"></p>
+        <p>Inline img <img src="#{cdn_url}" data-base62-sha1="#{upload.base62_sha1}"></p>
+        <div>
+          Block img <img src="#{cdn_url}" data-base62-sha1="#{upload.base62_sha1}">
+        </div>
         <p><a href="#{upload.short_path}">some attachment</a></p>
         <p><a class="attachment" href="#{upload.short_path}">some attachment</a></p>
         <p><a href="#{upload.short_path}">some attachment|random</a></p>