The former depended 100% on the latter anyways and there
was no clear separation of concerns between the two. For
example ComponentStream had methods to close a dialog.
Thus it made sense to merge the two.
Reuse the same controller that was used for adding children,
`WorkPackageChildrenRelationsController`, and renamed it to
`WorkPackageHierarchyRelationsController`. The difference is done with a
`relation_type` parameter which can be `"parent"` or `"child"`.
Reuse the same dialog that was used for adding children,
`AddWorkPackageChildDialogComponent`, and renamed it to
`AddWorkPackageHierarchyDialogComponent`.