Fix the code resolving OWNER role to correctly handle the situation
where the target model has multiple "belongsTo" relations to the User
model.
Introduce a new model setting "ownerRelations" that enables the new
behavior. When "ownerRelations" is set to true, then all "belongsTo"
relations are considered as granting ownership. Alternatively,
"ownerRelations" can be set to an array of the relations which
are granting ownership.
For example, a document can "belongTo" an author and a reviewer,
but only the author is an owner, the reviewer is not. In this case,
"ownerRelations" should be set to "['author']".
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