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HTML from govspeak on HTML attachments went out yesterday, we've had a couple support tickets from users who are seeing out-of-date versions of their HTML attachments.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 This prevents out-of-date HTML being shown in the period before the job to re-compute it has had time to complete.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.
if i understand this correctly, when there are validation errors, following which the editor chooses to not save, we will still reset the computed html. hence, `before_save` might be the better callback to use here.