Validate only govspeak fields for safe html

www.agileplannerapp.com/boards/173808/cards/5154

looking at the HTML validator in govspeak suggests
that it is useful to validate that HTML generated
from govspeak is safe. however, these checks seem
unnecessary for non-govspeak fields, because rails
ensures that safe html is rendered, and we take
care to not mark user input as `.html_safe`.

this is blocking editors from adding html in the
edition notes, which should be allowed.
Merge pull request #236 from alphagov/validate-only-govspeak-fields-for-safe-html

Validate only govspeak fields for safe html