Why this change?
Previously, we were preloading the necessary metadata for
`adminCustomizeThemes.show.schema` route in the
`adminCustomizeThemes.show` route. This is wasteful because we're
loading data upfront when the objects setting editor may not be used.
This change also lays the ground work for a future commit where we need
to be shipping down additional metadata which may further add to the
payload.
Why this change?
The `/admin/customize/themes/:id/schema/name` route is a work in
progress but we want to be able to start navigating to it from the
`/admin/customize/themes/:id` route.
What does this change do?
1. Move `adminCustomizeThemes.schema` to a child route of
`adminCustomizeThemes.show`. This is because we need the model
from the parent route and if it isn't a child route we end up
having to load the theme model again from the server.
1. Add the `objects_schema` attribute to `ThemeSettingsSerializer`
1. Refactor `SiteSettingComponent` to be able to render a button
so that we don't have to hardcode the button rendering into the
`SiteSettings::String` component
