certs: /request_cert: add_cert() strips HTML

* HTML in the grade range label was getting passed through to the
  certificate agent via xqueue. This strips HTML before passing labels
  through.

  This change is being rolled into my PR to master for the /request_cert
  endpoint feature.
Merge pull request #3030 from edx/jrbl/fix_ncp101

certs: /request_cert: add_cert() strips HTML