Allow links and images on legislation drafts

Note we're using a new sanitizer. Ideally we'd reuse the
`AdminWYSIWYGSanitizer`, but then code that would be correctly shown by
markdown-it (like the <h1> tag) wouldn't be shown on the web, which is
confusing. Ideally we would configure markdown-it to only allow the tags
present in the `AdminWYSIWYGSanitizer` and provide some kind of help
showing which tags are allowed.

app/helpers/application_helper.rb

@@ -25,7 +25,7 @@ module ApplicationHelper
       superscript:        true
     }
 
-    sanitize(Redcarpet::Markdown.new(renderer, extensions).render(text))
+    AdminLegislationSanitizer.new.sanitize(Redcarpet::Markdown.new(renderer, extensions).render(text))
   end
 
   def wysiwyg(text)