Fix re-accepting or re-rejecting remote group shares

When accepting a group share, a sub-share entry is created which also
has a different id.

When accepting or rejecting the sub-share, simply update the "accepted"
flag instead of trying to re-insert the entry.

Adjust getShare to also properly validate group share membership
when called on a sub-share id.

Signed-off-by: Vincent Petry <vincent@nextcloud.com>