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.

lib/admin_legislation_sanitizer.rb

@@ -0,0 +1,9 @@
+class AdminLegislationSanitizer < WYSIWYGSanitizer
+  def allowed_tags
+    super + %w[img h1 h4 h5 h6]
+  end
+
+  def allowed_attributes
+    super + %w[alt src id]
+  end
+end