[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]

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]

• 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?