feat(plex): selective user import (#2188)

* feat(api): allow importing of only selected Plex users

* feat(frontend): modal for importing Plex users

* feat: add alert if 'Enable New Plex Sign-In' setting is enabled

* refactor: fetch all existing Plex users in a single DB query

Co-authored-by: Ryan Cohen <ryan@sct.dev>

src/context/SettingsContext.tsx

@@ -22,6 +22,7 @@ const defaultSettings = {
   enablePushRegistration: false,
   locale: 'en',
   emailEnabled: false,
+  newPlexLogin: true,
 };
 
 export const SettingsContext = React.createContext<SettingsContextProps>({