[DISCUSS] 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.

i'll appreciate inputs from those who've worked on this area earlier to validate our understanding before we merge. git suggests: @jamiecobbett
I think it's reasonable to require that this is only used on a class that defines `GOVSPEAK_FIELDS`.  Having said that, having the `|| []` is a little misleading as it implies that this will fall back to empty array if the constans isn't defined.  Probably better to remove the fallback and make the requirement explicit.