Implement an genuine override method for templates

[rob-metalinkage]

Current approach to applying a skin is to overwrite templates from core - this means extra understanding, discipline and effort juggling git rollbacks if developing in core, and also requires an overwriting process during install.

it would be better to use an environment variable to specify a preferred location for templates etc and fall back on the defaults in core if not found. Preferably the same variable could be used for locating plug-in code rather than mutating core code.

[nicholascar]

I'm not having any luck applying Flask - the Python web API framework in use - with more than one template location.