Add API to undelete delete group shares

When a group share is deleted we keep track of this in the DB.
Right now it is only possible for a recipient to get back the share by
asking the sharer to delete it and to share it again. This doesn't
scale.

This endpoint makes it possible to get back the share.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>