Separate publication manifest for testing and production

If the app is built in DEBUG mode, use an alternative Shelf Manifest URL.

This is required once you've got your app published on the store to test & proof-read new issues. Place them first in the testing publication, then once they pass QA, insert them into the production publication for the rest of the world to see.

(NB: if all issues in this testing publication are free, checking through Apple is bypassed and you don't need to create new records in iTunes Connect.)