impl shipitscript method that determines most recent shipped rev

[lundjordan]

I suppose this will poll product-details? Most recent release in shipit with triggered ship phase?

It's needed to contrast against the treescript task that will check for the most recent "good rev" to ship from, If the most recent good rev in a repo is newer than the most recent shipped rev, we are good to create a new release based off of it.

[tomprince]

I don't think this logic belongs in a scriptworker, it should be in the action task that is triggered by cron direcly. This only needs read-only access to shipit, so does not need to be protected by chain-of-trust.

There is the issue that the shipit api is behind the VPN, but we should address that by making a read-only version available outside the vpn.

[MihaiTabara]

This is tracked in #93

[MihaiTabara]

Done in the aformentioned PR.