Changed Community page model to include placeholderfield 

Placeholders are special model fields that django CMS uses to render user-editable content (plugins) in templates. That is, it’s the place where a user can add text, video or any other plugin to a webpage, using the same frontend editing as the CMS pages. 
This seems to be more simpler instead of using cms page model since we just want front end editing.
Django-cms allows front end editing  on placeholder fields or cms-page only for staff members having required permissions. In any case either we use Placeholder fields or cms page model , we have to allow front end editing for non staff members 

Is there a specific reason to import `get_user_model` and `Permission` and not use it?