Add an always-present "user" skin

[peppy]

A user skin needs to be present at a system level (non-deletable) to allow users to set overrides that apply regardless of their skin choice. This is offered as a final layer that does not make permanent changes to the skin which is selected.

[peppy]

The concept of a user skin is no longer as we are taking different direction with skinning internally. Closing for now (and probably forever).