users and groups:
* No: Every user can invite other users and groups (default and old
behavior)
* Groups: other users can allways be invited, only groups require an invite_grant
* Users + groups: inviting both allways requires an invite grant
One need to keep in mind, that setting an invitation ACL via a group,
gives each groupmember the right to invite the group / create a group
event. So the last option propable only works, if users manage
invitations grants on their own, or admin only sets it in small working
groups, where every member is allowed to invite the whole group.
--> calendar backend code removes participants a user is not allowed
to invite
- new "only groupevents" filter, showing only real groupenvents not
events of groupmembers (added tooltips to explain filters)