closes: https://github.com/TryGhost/Ghost/issues/13739
- Ghost cannot write to the core folder in correctly configured production installations
- Built assets therefore need to be written to the content directory
- Ghost does not overwrite anything in the content folder as part of an upgrade, therefore static files that are provided by Ghost
must still live inside /core
- So as a result, we now have core/frontend/public and content/public
