enh(sharing): enable unsharing for sharees for DAV shares

[miaulalala]

Allow unsharing for a sharee via "Unshare from me" in the calendar dialogue. AFAIK Contacts doesn't have an unshare from me button, maybe something for the frontend devs.

How to test

Scenario 1

1. Create a group BobsGroup and adds Bob and Eric.
2. User Alice creates a calendar "Workouts" and shares it with BobsGroup (with edit permission).
3. Bob unshares the calendar "Workouts" from himself

Expected results:

1. 2 rows in oc_dav_shares like this:
   
2. Bob cannot see "Workouts" any more
3. Eric can still see workouts
4. Alice can see the groupshare

Scenario 2

1. Create a group BobsGroup and adds Bob and Eric.
2. User Alice creates a calendar "Workouts" and shares it with BobsGroup (with edit permission).
3. User Alice also shares the calendar "Workouts" with Bob directly:
   
4. Bob only sees one share and unshares the calendar "Workouts" from himself

Expected results:

1. 2 rows in oc_dav_shares like this:
   
2. Bob cannot see "Workouts" any more
3. Eric can still see workouts
4. Alice can see the groupshare, but not the direct share

Ref #1653

[github-advanced-security]

Psalm found more than 20 potential problems in the proposed changes. Check the Files changed tab for more details.

[st3iny]

Tested and works.

[ChristophWurst]

Tested & works