Why this change?
Instead of dealing with a generic object for `Theme#settings`, we want
to always be dealing with `ThemeSettings` objects.` Previously, we
converted the generic objects to `ThemeSettings` objects in the theme's
adapter `afterFindAll` function. This is not correct because updating
or saving the theme individual reverted the `Theme#settings` back to an
array of generic object.
To fix this problem, the proper way with our REST models is to overwrite
the static `munge` method and create `ThemeSettings` instances there.
Why this change?
On the `/admin/customize/themes/<:id>` route, we allow admins to edit
all settings via a settings editor. Prior to this change, trying to edit
and save a typed objects theme settings will result in an error on the
server.