Bump govuk_content_models to 5.0.0

This adds the attachment support for video editions
Correctly handle Panopticon registration in batch publish

This would break when an edition was published but
Panopticon timed out, as the edition could not be
published a second time to resend the notification.

We now fallback to invoking the registration method
again if the edition was published in a previous
attempt.

We think this only worked previously due to a bug
in the state_machine gem which was fixed in 1.2.0
(bumped with content models in previous commit).
Bump api adapters to 5.3.0

This includes the Asset Manager api adapter
Initialize an asset manager api client
Add caption file upload field to video editions
Rewrite the 'jumps to correct page' test so it isn't flaky

The existing test would often fail as the updated_at
times were often identical, making the order in 
which the results would be returned unreliable.
Merge pull request #95 from alphagov/attach-caption-files

Attach caption files to video editions