Adjust some of the other components like ManagedByToolbar or ManagedByAddModal to do a single concern

[carma12]

The interface of the ManagedByAddModal is kinda hard to understand when we don't know the implementation.

E.g. I'd expect to have something like:

<AssociationAddModal
   chosenEntries={managedByHosts}, // containing list of { key: host.uid, value: hostName }
   availableEntriesLoader={functionToProvidePromiseOfListOfAvailableHosts} // promise would return same data structure as ^^
   onSelectionChange={callback(selectedEntries)}
   title="string"     
/>

This would be usable for all the simple associations.

Originally posted by @pvoborni in #67 (comment)

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