Bug 11943: Prevent double accepts in Koha::Virtualshelfshare

Bug 11943: Prevent double accepts in Koha::Virtualshelfshare

This 'bug' existed long enough now to finally remove it ;)

We do so by deleting the invitation if the borrower already has a share
on this list. Actually not that hard.

We still need: a unit test and a db revision.

Test plan:
[1] Share a list. Let user B accept.
[2] Without this patch: Share again and let B accept again.
[3] Verify that you have two shares for this list in virtualshelfshares.
[4] With this patch: Share another list, let B accept.
[5] Share this other list again, let B accept again.
[6] Verify that virtualshelfshares does not contain double entries now.
    (Note: This pertains to the second list only.)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>