Add support for <sup> formatting element

This is based of 3e095cab

Related: pleroma#1191

app/javascript/flavours/glitch/styles/components/status.scss

@@ -81,6 +81,11 @@
       text-align: sub;
     }
 
+    sup {
+      font-size: smaller;
+      vertical-align: supper;
+    }
+
     ul, ol {
       margin-left: 1em;
 

app/lib/sanitize_config.rb

@@ -39,7 +39,7 @@ class Sanitize
     end
 
     MASTODON_STRICT ||= freeze_config(
-      elements: %w(p br span a abbr del pre blockquote code b strong u sub i em h1 h2 h3 h4 h5 ul ol li),
+      elements: %w(p br span a abbr del pre blockquote code b strong u sub sup i em h1 h2 h3 h4 h5 ul ol li),
 
       attributes: {
         'a'          => %w(href rel class title),