Implement right sidebar option as user option

Currently an admin can set the "sidebarRight" option, but the user is unable to change it for themselves.

This option felt like a natural fit, and I currently use it already on my instance.

src/modules/config.js

@@ -54,7 +54,7 @@ export const defaultState = {
   interfaceLanguage: browserLocale,
   hideScopeNotice: false,
   useStreamingApi: false,
-  sidebarRight: false,
+  sidebarRight: undefined, // instance default
   scopeCopy: undefined, // instance default
   subjectLineBehavior: undefined, // instance default
   alwaysShowSubjectInput: undefined, // instance default