Forward port: Support Channel Manager Publication Event through HippoEventBus

The channel update event [1] is too difficult to detect the channel publication event (when user clicks on publish button).

[1]: http://www.onehippo.org/library/concepts/channels/channelmanagereventlistener.html

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Woonsan:

Wouldn't it be reasonable if MountResource#publishChangesOfUsers(List<String>) simply publish an event with publication info to HippoEventBus? Then, most use cases will be sufficed for now and less risky to add it
even in 7.9.