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HTML from govspeak on HTML attachments went out yesterday, we've had a couple support tickets from users who are seeing out-of-date versions of their HTML attachments.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 This prevents out-of-date HTML being shown in the period before the job to re-compute it has had time to complete.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.
@alext: even i expected that `body_changed?` would be false after a `save!` till the time i wrote a test, and did some debugging. it seems the condition provided is evaluated in the correct context of attributes, and [it works](https://github.com/alphagov/whitehall/pull/1769/files#diff-9cce8bf2f9f9254c9f9a427fa6db7e67R31). tried digging into `Callback` internals to figure out how that happens, but couldn't precisely locate it.