Allow select/deselect all in people table

[hernantz]

Next to the "thing" column in the people table add a checkbox.
Following rules should be considered:

• If everyone is selected for that item, then this checkbox must be checked
• If someone is not selected for that item, then this checkbox must not be checked
• When toggling the checkbox to checked, everyone should be assigned to that item.
• When toggling the checkbox to unchecked, everyone should be unassigned from that item.