Suppress group transmission to managed server

[MarcelWaldvogel]

Add a new option to the "managed server" page to disable transmission of group information (including group memberships of any user connecting to the XMPP server, and other members of the groups the connecting user is a member of).

Concept:

• Selection option "Transmit group memberships" (active by default?)
• This setting is saved in the global app-specific settings
• When this option is set, the sharedRoster() function in lib/Controller/ExternalApiController.php always returns

[
  'result' => 'success',
  'data' => [ 'sharedRoster' => [] ]
]