Clean up Banzai HTML for HipChat

The `class` and `data-*` attributes are meaningless in HipChat, and it
would probably be better to limit the tags, too. For example, we could
avoid block-level elements in `render_line`.

app/models/project_services/hipchat_service.rb

 class HipchatService < Service
+  include ActionView::Helpers::SanitizeHelper
+
   MAX_COMMITS = 3
 
   prop_accessor :token, :room, :server, :notify, :color, :api_version
@@ -138,6 +140,7 @@ class HipchatService < Service
 
       html = Banzai.render(text, context)
       html = Banzai.post_process(html, context)
+      sanitize html, attributes: %w(href title alt)
     else
       ""
     end