Clear computed govspeak when content changes

Since the code that pre-computes HTML from govspeak on HTML attachments went out yesterday, we've had a couple support tickets from users who are seeing out-of-date versions of their HTML attachments. To prevent this happening, we clear the computed values when changes are made. This prevents out-of-date HTML being shown in the period before the job to re-compute it has had time to complete. This means that users will always see the latest version of their HTML Attachment, as it gets generated on-the-fly when a pre-computed version is not yet available.
Agreed.  after reading http://stackoverflow.com/questions/14804415/what-happens-between-after-validation-and-before-save it would seem that `after_validation` is called even when validation fails, so this isn't what we want.