Never set cache TTL to less than 5 seconds.

In order to avoid hammering origin, the cache TTL is always set to at
least 5 seconds, which will still allow for timely publishing.
Remove auto-cleanup of intents on publish

Now that we're setting a minimum cache TTL of 5 seconds, there's no
pressing need to clean these up immediately.

This behaviour previously meant that it was important that any "coming
soon" content item was published before the intent.  Removing this
behaviour simplifies this use case, and also simplifies the code.

Intents will now only be cleaned up by the nightly cronjob.
Merge pull request #95 from alphagov/rejig_intent_cleanup

Apply minimum cache TTL of 5 seconds, and remove publish intent auto-cleanup.