Support adding enforced configs to themes

Problem/Motivation

We can currently add enforced configs to both modules and profiles. However, some configuration objects should be saved to themes. We cannot currently target themes.

Steps to reproduce

On the Config Enforce settings page, no themes are listed among the available "modules".

Proposed resolution

The concept of "Target Module" should perhaps be renamed to "Target Extension". We can then, presumably use ThemeHandler to do much the same as we do currently with ModuleHandler. That is, we can pull up a list of themes, look up their filesystem paths, etc.

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD