Keep information of invited groups

[martinbu]

This is part 3/3 of the issue #1268

Is your feature request related to a problem? Please describe. With #902 it is now possible to add contact groups to the poll. To invited the contacts inside the contact group, the group has to be resolved to individual shares. After this action the information of the invited group is lost.

Describe the solution you'd like List contacts of contact groups as sub items of the group to keep the information which groups were invited (in both "unsent invitations" and "effective shares"). Add a Button to send an invocation mail the the whole group and to each single contact.

It would be great if the following behaviour could also be implemented.

1. If a contact is in multiple groups, this should be detected and the contact should only be invited once.
2. So if a group or a single contact that is in a group is invited, the contact should be removed from all groups in the unsent invitations list.
3. But in the list of the already invited contacts, the contact should show up in all invited groups.