DEV: Open theme settings objects editor from admin customize theme page (#26006)

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

app/assets/javascripts/admin/addon/components/site-setting.hbs

@@ -25,16 +25,25 @@
 </div>
 
 <div class="setting-value">
-  {{component
-    this.componentName
-    setting=this.setting
-    value=this.buffered.value
-    validationMessage=this.validationMessage
-    preview=this.preview
-    isSecret=this.isSecret
-    allowAny=this.allowAny
-    changeValueCallback=this.changeValueCallback
-  }}
+  {{#if this.settingEditButton}}
+    <DButton
+      @action={{this.settingEditButton.action}}
+      @icon={{this.settingEditButton.icon}}
+      @label={{this.settingEditButton.label}}
+      class="setting-value-edit-button"
+    />
+  {{else}}
+    {{component
+      this.componentName
+      setting=this.setting
+      value=this.buffered.value
+      validationMessage=this.validationMessage
+      preview=this.preview
+      isSecret=this.isSecret
+      allowAny=this.allowAny
+      changeValueCallback=this.changeValueCallback
+    }}
+  {{/if}}
 </div>
 
 {{#if this.dirty}}

app/assets/javascripts/admin/addon/components/site-settings/string.hbs

@@ -1,11 +1,5 @@
 {{#if this.setting.textarea}}
   <Textarea @value={{this.value}} class="input-setting-textarea" />
-{{else if this.setting.json_schema}}
-  <DButton
-    @action={{fn (mut this.showJsonEditorModal) true}}
-    @icon="pencil-alt"
-    @label="admin.site_settings.json_schema.edit"
-  />
 {{else if this.isSecret}}
   <Input @type="password" @value={{this.value}} class="input-setting-string" />
 {{else}}
@@ -13,14 +7,4 @@
 {{/if}}
 
 <SettingValidationMessage @message={{this.validationMessage}} />
-<div class="desc">{{html-safe this.setting.description}}</div>
-
-{{#if this.showJsonEditorModal}}
-  <Modal::JsonSchemaEditor
-    @updateValue={{fn (mut this.value)}}
-    @value={{this.value}}
-    @settingName={{this.setting.setting}}
-    @jsonSchema={{this.setting.json_schema}}
-    @closeModal={{fn (mut this.showJsonEditorModal) false}}
-  />
-{{/if}}
+<div class="desc">{{html-safe this.setting.description}}</div>