Design improvments: Permission childs aligned to top; link-button
Permissions-link bugfix on user/edit_others view
Added permissions index-page, added switch button for users

Switch button: Switches between user-level and group-level
index-page:
- link to groups-managment
- link to users via entering user-url
- explanation of the permissions-system
Simplified and fixed bug for has_user_permission

by using has_user_specific_permission instead of using other more
difficult code.