ejabberd module to prevent OMEMO sessions from being established

 Unless the configured access rule (called 'omemo' by default) returns
 'allow', this module prevents OMEMO sessions from being established.
 Requests to query the devicelist from other users are rejected. Requests
 to publish a devicelist are also rejected, and all OMEMO nodes referenced
 in that devicelist are removed. Incoming devicelist updates are silently
 dropped. OMEMO-encrypted messages are bounced with an error message.