Make audited associations extendable from plugins
audited can't be called twice in the same model, plugins can't easily extend existing
audited definition in core. For attributes this is not a huge problem as we always use
:except so all attributes added from plugins are audited but associations, that are listed explicitly can't be added. Since Foreman adds this using DirtyAssociations module, it should also provide a way to add more associations from plugins.