Classifieds phase 1

- [x] - Backend to manage classifieds
- [x] - Filtering by specified category, status (active, inactive)
- [x] - Reviewing new classifieds
- [x] - Email notifications when new classifieds are posted, delivered to an address entered in the plugin
- [x] - Managing classifieds is included in user rights options, users with this option activated can edit classifieds, set them to inactive or delete them.
Available permissions:
 - delete
 - add - (add categories)
 - edit
 - settings - (manage settings)
 - access - (access to plugin - when unset, user cant enter plugin at all, proper message will show up)
 - activate - (activate classifieds)
 - deactivate - (deactivate classifieds)
- [x] - Remove permissions when plugin is uninstalled
- [x] - Render custom page and return message when AccessDeniedException is thrown