Add ability to disable alerts from (pipeline, searches) combinations

Followup from the semi-regular RRT call of Tuesday May 23.

It seems we currently have the ability to disable alerts from individual pipelines, but not from (pipeline, search) combinations. I would like to request the latter ability as well. The use case is that we could in principle have problems from e.g. PyCBC Live early-warning, but not from PyCBC Live full-bandwidth.

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