Werk #6226: Allow to match against the monitoring site in rules

This allows to apply rules only on specific sites. Restricting rules to sites can yield performance benefits in environments with many sites. Beyond that, this is required to make the feature expect regular messages work in a distributed environment with config replication.

Example: If you have three centrally configured event consoles, and you want a device to regularily report to one of them, you previously had no way to configure that only the one site reports missing messages. Instead, all sites the device doesn't send messages to would repeatedly open events. This limitation can now be worked around by specifying to which site expect rules should apply.

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