Release 7.4.7 (lmn74)

 intranet, webfilter, printing) no longer shells out to `sudo
 /usr/sbin/lmncli mgmtgroup ...`; it now calls linuxmuster-api
 (POST/DELETE /v1/managementgroups/{group}/members).

• [lmn_users] Assigning/unassigning a parent to a student no longer
  shells out to /usr/sbin/lmncli student --add-parents/--remove-parents;
  it now calls linuxmuster-api (POST/DELETE /v1/users/{student}/parents).
• [lmn_common] lmnapi_client: add add_management_group_members(),
  remove_management_group_members(), add_parent(), remove_parent().
  Fix response handling accepting only HTTP 200 (other 2xx codes were
  treated as errors) and crashing on a body-less response.