Add a "report" entity to allow for federated report over pods

[Flaburgan]

With spam starting to increase, diaspora needs a way to forward report to the author's pod, the only one who has the power to remove the content on the whole network. See diaspora/diaspora#7316 for details on the feature. I'm opening an issue on this repository as asked here.

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[tclaus]

This is a very useful feature.