Allow deletion of future group memberships

[rouven0]

When planning a group membership that has a start date in the future, there is currently no intuitive way to cancel that membership. The only workaround existing in the UI is to set both start and end date of the membership in the past. Nonetheless, this will lead to the group membership existing in the group history of the affected user.

[lukasjuhrich]

We can reuse end_membership here and just show a different button (the trash button)