Event carousel route on instance level

My work on embedding the event carousel has been mostly chaotic. There have been several pull requests and single commits regarding either embedding in general (e.g. #802), the event stream (e.g. #794, #797) or the combination of both (e.g. #796). This adds another pull request to the list.

This deprecates the instance activity route in favor of the event controller. This means less code duplication and more functionality.

Note that I interpreted the `event.index_all` permission as meaning *all* events. So you do not need that permission in order to see all events from a single instance. Not sure about that however.