Improvement for the Rules integration

I'm moving the discussion from the original issue, so it won't go in the middle of a (almost) committed patch - #298109: Rules integration.

I don't think that dynamically creating flags is a common use case - I don't see why one would need that. But yes, it's not possible right now.

Use case: Think of groups, where every group has it's own administrator of spam.

One more thing with the current implementation: If I try to create an action to flag a user when there's no user-flag-type, I get an error message telling me this can't be done. If we would use the flag as a regular argument, then this action would be disabled and appear in the "X action actions are not configurable."