Add alerts task

This peels off the work in !852 (merged) related to the alert task specifically (as opposed to plugging the alerts task into the orchestrator).

I've bumped the version of fastavro to be at least 1.6.1, because 1.6.0 adds support for a strict option in the validate function that we use in the validation unit test, and 1.6.1 fixes a bug in that strict option.