Allow "class" attribute on the "a" tag in sanitization (#3623)

This preserves `<a ... class="u-url mention">` from other Mastodon instances.

app/lib/sanitize_config.rb

@@ -8,7 +8,7 @@ class Sanitize
       elements: %w(p br span a),
 
       attributes: {
-        'a'    => %w(href rel),
+        'a'    => %w(href rel class),
         'span' => %w(class),
       },