Remove markdown from hints; add optional flag
Remove markdown from hints; add optional flag

Hints are getting the markdown filter applied in question templates.
Also, an `optional` block has been added to question templates, so
might as well pass it through if it exists.

alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend