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
Why this change?
The logic for validating a theme setting's value and default value was
not consistent as each part of the code would implement its own logic.
This is not ideal as the default value may be validated differently than
when we are setting a new value. Therefore, this commit seeks to
refactor all the validation logic for a theme setting's value into a
single service class.
What does this change do?
Introduce the `ThemeSettingsValidator` service class which holds all the
necessary helper methods required to validate a theme setting's value
