Configurable media browser path

[jimafisk]

Currently we put all media files in a media/ folder, which makes them available at URLs like: www.mysite.com/media/myfile.png.

It's safe to assume that some people might want a public media library of their own to live at that URL, but that couldn't do that currently because of the name conflict.

The media/ folder should remain unchanged in the project structure for consistency, but we should allow an override in plenti.json that changes the subfolder for these files in the built output so a user could occupy the /media URL for their own purposes.