enable ejabberd OMEMO encryption in groupchat

Created by: mray

I can't use OMEMO encryption in groupchats on my FreedomBox. When creating a groupchat on my freedombox Server with my android app conversations it reports inside the groupchat:

XEP-0313: MAM ---> no

My guess is that XEP-0313 is required but not enabled on FreedomBox.