As I mentioned [here](vikunja/api#1442 (comment)), this is mainly a cleanup of @zewaren 's original [PR](vikunja/api#1442).
It adds support for the `RELATED-TO` property in CalDAV's `VTODO` and the `RELTYPE=PARENT` and `RELTYPE=CHILD` relationships. In other words, it allows for `ParentTask->SubTask` relations to be handled supported through CalDAV.
In addition to the included tests, this has been tested by both @zewaren & myself with DAVx5 & Tasks (Android) and it's been working great.
Resolves vikunja/api#1345
Co-authored-by: Miguel A. Arroyo <miguel@codeheads.dev>
Co-authored-by: Erwan Martin <public@fzwte.net>
Reviewed-on: vikunja/api#1634
Reviewed-by: konrad <k@knt.li>
Co-authored-by: Miguel Arroyo <mayanez@noreply.kolaente.de>
Co-committed-by: Miguel Arroyo <mayanez@noreply.kolaente.de>
