[Do not merge] Unique constraint for duplicate child shares

[IljaN]

For Debugging

CREATE UNIQUE INDEX child_share_unique ON oc_share (share_with, parent) WHERE parent IS NOT NULL AND share_type = 2

[update-docs[bot]]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[IljaN]

• [ ] Build equivalent index for MariaDB/MySQL as most of the acceptance tests run with those.

[IljaN]

• [x] Analyze if creating an index on the "parent" column makes sense as the unique-index uses a WHERE. Will this improve INSERT performance? Are indexes used by other indexes?

[DeepDiver1975]

cleaning up - I think this is no longer of anybody's interest