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out-of-date versions of their HTML attachments.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 This prevents out-of-date HTML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.
Clear computed HTML on before save

This ensures that validation failures don't
end-up unnecessarily clearing the computed
HTML.
Merge pull request #1769 from alphagov/clear-computed-govspeak-html

Clear computed govspeak when content changes