Core/RBAC: Simplify RBAC implementation
- Drop groups (roles than can have inherited roles) and roles (set of
  permissions)
- Permissions can now have inherited permissions (those act as roles)

RBAC DB structure is now limited to four tables
- rbac_permissions: Contains permissions and roles
- rbac_linked_permissions: Contains the relation between permissions and
  linked permissions (those permissions that have linked permissions are
  called roles)
- rbac_default_permissions: Contains the list of permissions to be granted
  to each security level [Added to maintain compatibility in an easy way]
- rbac_account_permissions: Contains the list of permissions granted or
  denied for a particular account.

NOTE: IF YOU ARE USING CUSTOM PERMISSIONS, ROLES OR GROUPS CHECK THE SQL
BEFORE APPLYING...