user-account: expose membership of user groups to users.
- for review rules this might come in handy
- it's not a secret information about which groups user belongs too
authentication: introduce a group sync flag for plugins.
- we'll skip any syncing on plugins which simply don't get any group information
- we let plugins define if they wish to sync groups
- prevent from odd cases in which someone sets user groups as syncing, and using regular
plugin. In this case memebership of that group would be wiped, and it's generaly bad behaviour.