Omit redirects to content-app

[dralley]

Manifest data is now available on the content unit model as opposed to needing to separately redirect to the content app. That means that in theory we should be able to eliminate this redirect entirely and serve the data directly from the registry.

Unfortunately there seems to be some issues with the caching machinery getting in the way of this.

See the discussions on previous PRs:

#1890 (comment)
#1644