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HTML from govspeak on HTML attachments went out yesterday, we've had a couple support tickets from users who are seeing out-of-date versions of their HTML attachments.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 This prevents out-of-date HTML being shown in the period before the job to re-compute it has had time to complete.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.
I guess it's a difference between then the callback chain is setup, and when it's evaluated.  I suspect that if we moved the conditional inside the method, it would behave differently, which is... non-obvious.  Oh well, as long as we have test coverage, and it works, then all is good.