Improve event message rules UI

[bjendres]

The more message rules there are in an event, the more difficult it becomes to see what's actually happening. And with #39 it would potentially get a lot worse.

Therefore we suggest a new interface where you have multiple filters for your rules that work in the same way as the actual sending out mechanism would. So there would be (optional) filters for old status, new status, role and language (see #39).

Remarks:

1. It's important, that the order of the message rules is retained, so the user can see what exactly would happen.
2. It should be possible to create this functionality with SearchKit, maybe requiring a couple of adjustments in the BAOs.
3. It should be possible to re-order the rules in this view.
4. It would be important that this view/SearchKit could be used for an event and an event template. This is technically the same thing, but the button to trigger this might have to be in different places.