FEATURE: Theme settings (2) (#5611)

Allows theme authors to specify custom theme settings for the theme. 

Centralizes the theme/site settings into a single construct

app/assets/javascripts/admin/components/theme-setting.js.es6

@@ -0,0 +1,9 @@
+import BufferedContent from 'discourse/mixins/buffered-content';
+import SettingComponent from 'admin/mixins/setting-component';
+
+export default Ember.Component.extend(BufferedContent, SettingComponent, {
+  layoutName: 'admin/templates/components/site-setting',
+  _save() {
+    return this.get('model').saveSettings(this.get('setting.setting'), this.get('buffered.value'));
+  }
+});