Fix underline

app/lib/text_formatter.rb

@@ -175,6 +175,7 @@ class TextFormatter
         fenced_code_blocks: true,
         strikethrough: true,
         superscript: true,
+        underline: true,
         highlight: false
       )
     @markdown.render(html)
@@ -197,6 +198,10 @@ class TextFormatter
       "<p>#{text}</p>"
     end
 
+    def underline(text)
+      nil
+    end
+
     private
 
     def process_program_code(code)