API Replaced SSViewer.custom_theme with SSViewer.theme_enabled

Since we can't influence the setting of configuration values,
we also can't set/unset the 'custom_theme' value based on which
theme is set. This means the 'custom_theme' value goes stale,
and we can't rely on it e.g. in FilesystemPublisher.

The 'theme_enabled' toggle is a cleaner solution to the same problem,
since the 'custom_theme' was really just a way to remember the original
theme, while still disabling it. The toggle makes this more explicit,
but also requires users of the 'theme' setting to check for it.

Depends on https://github.com/silverstripe/silverstripe-cms/pull/307