core.cms: improved admin for menu items #174
* show only root items
* nested items are shown as inlines
* reordering nested items easier
core.cms: improved caching - #174

* added cache_by_group feature (ACL groups)
* added drf-extensions in requirements
* separated cache invalidation between Pages and MenuItem

related to #174
UI: refactored to reflect changes to core.cms #174

* login & logout in User model
* custom login, logout, loggedin and loggedout events
* refresh menu when logging in & out