CalDAV sharing permissions should not depend on the order

[nickvergessen]

Steps

1. Create a calendar as admin
2. Create a user test1 that is part of group1 and group2
3. Share calendar with group1
4. Share calendar with group2
5. Tick the "can edit" box for group1

Expected

test1 should be able to create events and see the calendar in the normal list.

Actually

test1 sees the calendar in the subscription list and therefor can not add events.

Workarounds

Swap step 4 and 5 makes it work. But you can then again break it, when you remove the "can edit" from all shares and add it to the first group again....

PS: CardDAV has no adjustable test for this...

I'd like to backport this (apart from the first commit)

Fix #3677

[mention-bot]

@nickvergessen, thanks for your PR! By analyzing the history of the files in this pull request, we identified @schiessle, @tcitworld and @rullzer to be potential reviewers.